If the chest measurement is correct for the 3T size, you could use safety pins to pin the sweater together and see how it fits. If the sleeves and chest are ok, how does the length look?
If it seems out of proportion for your son, you might consider ripping out and shortening it. I'm afraid that if you lengthen the sleeves and the chest is still a 3T, you'll end up with a sweater with disproportionately long sleeves.
It's a very good looking pattern and well worth getting it to fit.