Maple Syrup Urine Disease is one of the things they test for in the CA Newborn Screening blood test. For you other moms not in California this is a heel-stick blood test that is done the day or two after they are born while still in the hospital. It tests for all sorts of things, including blood disorders and metabolic disorders. So he should have been already tested for that one, at least preliminarily so. Did anything show up on Newborn Screening? Your pediatrician would have/should have notified you.
I find that peed-on diapers in general smell a little maple-y. Disposable diapers that is.
If you think it is not normal, call your ped.