H.B.
3 1/2 is a little old for needing a nap at noon in my experience. Usually it is about age 3 or 3 1/2 that most kids get rid of the am/mid nap and may still take an afternoon nap, but MANY don't require a nap at all anymore at that age....that is unless they have done something really tiring for a morning activity, etc.
You really can't FORCE anyone to sleep that isn't tired. You can though get him in the routine of 'resting' or doing something quietly for 30-45 min each day while his brother naps. He can look at books, color, play play-doh, blocks, cars, watch a short show, etc... This will give him some down/rest time while you get a break too. It won't be the same as when they both napped together, but you didn't think that would last forever anyway right?
Keep in mind that the more activity, exercise, mental stimulation and play he has had all morning will have a HUGE direct impact on hiw well the 'quiet/rest' time goes.... he may even take a brief nap if he's had enough energy expended! Good luck and don't push him too hard....he is growing up and doesn't need that sleep time anymore!