Just nap, too.
Or do nothing.
Both my kids still nap. In the afternoon. When my daughter is not in school or on break, they BOTH nap.... at the SAME time.
I then nap too... or do nothing. That is MY time.
Try to get your kids... to both nap in the afternoon at the SAME time. This would be your baby's 2nd or 3rd nap. Your 3 year olds, nap. Too.
That is what I did with both my kids, once I had 2 kids. My son as a baby would nap at least 3 times a day as an infant.
In the morning when your baby is napping... spend some time doing something with your son... and then what 'chores' you have. Incorporate him into that... it is also good for him to learn, about "Teamwork" and "helping" Mommy. But that is what you are already doing.... which is what many Moms do. And what I do too.