We got Magnatiles when DD was 2. Since then, we've added more sets. She's 8 now, and its the only toy that she and her friends have played with consistently throughout the years. They get lots of use, and are the one thing that when kids come over, always get pulled from the shelves.
To put it in context, we also have a ton of Playmobil, lincoln logs, blocks, backyard trampoline, Calico Critters, Barbies, swingset, stilts, pogo stick, wooden blocks, matchbox cars, etc.
The Playmobil are also a hit, but not as much as the magnatiles. The Calico Critters are fun to get and play with a bit, but then they tend to get lost and forgotten.
Magnatiles are a great toy. Would he be frustrated that the magnets are weak and structures fall apart easily? They are easy to rebuild too, but some kids who come over to play can't seem to handle that aspect of them. I've explained it to my kids and it doesn't seem to be an issue for them now. We have the big mangatile set, and I'm not sure how fun it would be to have just the smaller one. The more tiles, the more the possibilities.
There are a lot of playmobil animals. My DD loves the animals too and plays with them a lot more than the people. Also, there is Littlest Pet Shop. I know it is marketed to girls, but they are animals. The tiny baby ones are cute.
Calico critters are cute and nice. They are pricey. A little fuzzy to the touch. The clothes sort of suck.
Does he have any lego? He's at a good age for lego building. He could build houses for all his random little animal friends.
What about a doctor's kit? My son enjoys playing doctor, too, and he will give everyone check-ups. Things with small pieces (Playmobil, Calico Critters, etc) drive me bonkers, as the accessories just get lost easily or create clutter.
My 4 year old loves his Legos, too. We got a big bin of them (like this) and he enjoys making buildings, cars, robots, etc. The bin is super helpful for clean-up, as all the small pieces are properly stored and easily found the next time. My son rarely ever plays independently, but will play alone with Legos.
I know you want to stay away from plastic, that has always been my goal too, but it sure gets harder as the kids get older. One toy that has really stood the test of time is our Hotwheels Garage. It gets played with EVERY day, which makes it worth having a big plastic thing in my living room!
My son got this set for Christmas, and it has been a hit. No super small pieces, so it's safe around our 17 month old. Lots of room for creativity, as the pieces fit together all sorts of ways. Our set is the car-building one, since he's a car fanatic, but they also have basic sets with more colorful pieces and no wheels. There are directions included for making certain types of cool vehicles, which he likes to try sometimes. Mostly, he likes to create his own.
Have fun shopping!
My inlaws got some magnet toys, not magnetiles, but Magformers, and my kids LOVE them! It is actually the one main toy that they are still playing with since Christmas!
My child HATES the building sets others have suggested and prefers plain blocks to all of them-won't even touch a lego. As a just turned 5 years old he LOVES LOVES his calico critters and the knock-off Target ones,http://www.woodzeez.com they get played together and we have none of the extras- we have real log/tree blocks and found items and he can spend tons of time with them.
There are these items - http://www.amazon.com/Learning-Resources-Pretend-Animal-Hospital/dp/B000NHNZQ6/ref=pd_sim_t_7
http://www.amazon.com/REALISTIC-STETHOSCOPE-Educational-LIGHTWEIGHT-SINGLE-HEAD/dp/B002PJEXEI we also love playing with our child size stethoscope in our home-I feel it's a must have toy
My kids love magnatiles. For one child, I think a 48 piece set would work. THe more you have, the bigger the structures become. My kids also love playmobil, but not really at 4. too many small pieces. Honestly, I feel the same about the calico critters. We have some of those, but little sister didn't really like playing with them until she was closer to 5. Don't get me wrong-- they're cute. Just small, with lots of small accessories, and difficult to dress.
Another idea, since you said he loves cats, is http://www.amazon.com/Manhattan-Toy-107790-Nursing-Nina/dp/B0001LATN6/ref=sr_1_1?ie=UTF8&qid=1358603469&sr=8-1&keywords=mama+cat+kittens I got this for a relative and it was loved.
How about Haba marble run? It's all wood, except for the marbles. My boys love watching marbles run around a track. They are kind of pricey, though.
What about building straws? http://www.amazon.com/Roylco-Inc-R-60880-STRAWS-CONNECTORS/dp/B003LMRLHE/ref=pd_sim_t_2
I never actually got these for my kids but it was on my wish list forever.
He might appreciate some duct tape if he likes taping the blocks together. It comes in handy with cardboard box building.
We had some Calico Critters before and I think they are okay but not great. They are really dolls with tiny clothes etc for playing in little houses and such.
My kids loved loved loved Schleich animals. They got played with day in and day out month after month year after year by everyone. There are adults and babies of the animals so they are nice for family groups. They live in block houses and travel in sand boxes and mud puddles etc. and ours have been used as big chess pieces too. I do not know how the girls kept track for that!