You have already received some very good advice. Especially, in trying to take your children to mass separately. Does your younger son have the same diagnosis your older one does? He may simply be following his brother’s lead. Separating them may work wonders.
What does their doctor say? He may have some behavioral tips ask him!
You may get some criticism, for splitting up the family during mass time. But, if their being together contributes to the problem in any way, isn’t it better than having them not go at all? Don’t be afraid of what others may think.
Right now, your goal for your younger son is getting him used to going to mass. For the older one, it is getting him to understand the mass, and getting him ready for his sacraments. Don’t lose sight of these goals!