Teach him to whisper.
Teach him it is quiet time.... and that means Mommy too and that it is your work time/quiet time too.
My kids, know that when it is quiet time, Mommy also "rests" and/or does my work. Its the ROUTINE.... that they know and we do. Everyday.
My son is 3.5 years old... and when when either of my kids are napping (or even Daddy/my Hubby), the kids just KNOW to be quiet and whisper.
They do it, unbelievably.
My son still naps though... are you sure your son does not get tired?
My son naps around 2:30. For 2 hours. Its doesn't affect his bedtime or night time sleep. But sometimes he naps earlier. I go by his cues.
Or, play white noise in your baby's room, when baby is napping, to drown out the noise your son makes.
A Leapster is also a great thing for a kid the age of your older son. And it keeps the child busy and it is educational. My son loves his and my daughter.... it is "quiet" when they are using it, and keeps them occupied for a good amount of time chunks.
all the best,
Susan