My brother is allergic to some fish and most nuts, and we had to go through this a lot when he was younger. He got a false result saying he was allergic to carrots and celery, and sunflower oil, etc. and it turns out he was never allergic to any of that stuff, but we had it out of the house for years. (He is anaphylactic to some things though - like fish). So make sure you have an accurate diagnosis.
There are a number of substitute butters available...
you also need to avoid bulk items, as bulk stores may use the same scoop or bin for nuts as well as all other products, and you can never be sure if anything is contaminated.
Chapman's icecream and Nestle Chocolate (do you have those in Maryland?) produce lines that are peanut/nut free. Nestle makes a bunch of different chocolate bars, and Smarties (which are like um... more colourful M&M's I guess). The packages that are produced in the peanut free plant have giant labels on them. I have heard that Nestle Products are harder to acquire in the U.S.? If you're ever in Canada though... Chapman's icecream makes all sorts of standard icecream flavours as well as popsicles and other frozen treats.
I hope the transition goes well and that you can find great substitutions for everything your son loves!