Yes, the mei tai would be a great option since he can ride on your back but I think the best thing might be the toddler patapum here www.patapum.com They are in Europe and go up to 65 lbs!
They don't have wagons where you are?? It's so funny when we think something is just universal and then find that it's not. Are you in Europe? If you look here: http://www.radioflyer.com/buy/howtobuy.html there is a list of international Radio Flyer distributors. Those are the best wagons. DO NOT buy any wagon with hard plasict wheels--the sound they make is deafening. You want rubber tires. Wow, if no one else has a wagon, this will make your ds the hit of kindergarten, lol!
I was wondering if it would be a good idea to get a bike trailor? We have one for our two and it's great for that type of trip. Dd is starting K4 next year and I'm planning to use ours to pick her up. Ds is 17 mos and he can ride in it too. It's quite a haul for me though. I know the exercise will be good for me though. If a trailor is too expensive how about a bike seat? Just a thought.