Depending on his age and how much you want to work on it, maybe he will decide to nap again. Or maybe you will have to decide to "make" him nap again.... My almost 4 year old can go either way. If she naps, she is up late, but much more well behaved and easy to have around until 9 or 10 pm. I will be up with the baby anyway, so I will probably still enforce the naps for her for a while... unless DH wants her down at 8 with the other 3 kiddos, but then baby will still be up, so we'll see about that.
Like I said, depending on his age, I might try something like this:
7am all wake up, unless baby wants to go back to sleep after final nursing.
Breakfast, outside play, games, puzzles, playdoh, etc. keep him VERY busy so he tires out.
Lunch right before nap. Maybe even buy him a new thing for his room, like a nightlight, new toy to sleep with, etc. Set the stage so he knows you mean business and that he needs to stay put in his bed, on the floor, where ever he sleeps. My 4 year old DD stays on the living room floor and naps while I eat in kitchen, sit on couch with laptop, etc. I have the other three kiddos in other rooms at this time. The 2 year old naps in my room and the 6 and 8 year olds play quietly in their room. I would get him down by 1 at the latest. Even if he tosses and turns for a while, he will probably settle down by 1:30, sleep until 3 or 3:30, then you just do a repeat of the morning and get him back down to bed at 9 or so....
Just my 2 cents, although I am DREADING being sleep deprived. I just weaned 2 year old DS 2 months ago and have been sleeping about 8 hours each night with 2 pee breaks in there and I have been loving that. Almost time to kiss that goodbye! But I do fully intend on having quiet time and hopefully SLEEPING time for myself every afternoon from about 1:30-3. Unless baby is up. The 2 year old and 4 year old will need to obey and stay in their spots and rest! ;)
HTH! And you will be fine! You'll figure it out.