I know it is hard to break that part where they fall asleep next to you....I have some thoughts:
1) Is his bedtime to early? If he is not tired, he will tolerate the routine, but then be ready to play.
2) Has he had bad dreams, or other negative associations that would make him not want to sleep?
3) To start weaning him, only lay down with him the one time. If he gets up, it is going to have to be his problem how he gets back to sleep.
This advice may not rest well with you, but you have unwittingly created a problem for yourselves. That is, children do better, in general (because there are always exceptions to the rule), when they 'learn' from a young age (infancy) to fall asleep by themselves. You have made your son dependent on your presence. To manage the current problem, if he does not cooperate with the routine, place him in time out for approximately ten minutes. Then, if he is ready to be in his bed quietly, he can return to bed. If not, he should remain in time out. Alongside this part of the plan, establish a reward system whereby he receives a modest reward the morning after he accomplishes the routine without difficulty. Now, you are the parents and will certainly decide what you think is best, but you would be doing your son (and yourselves) a favor by weaning him and his brother from the part of the routine whereby you are resting next to them.