Allergies do run in families. We are vegetarians and my ped said it would be a great source of protein when I asked about giving my nearly 2 year old daughter it for the first time....as long as we didn't have any history of nut allergies. She was fine with it and as someone else mentioned, we spread it thin to prevent choking. I was prompted to ask with my daughter because her best friend had been eating it since before she was 1. So, with my son, I was much less cautious and probably offered it to him not long after 1. (They are only 17 months apart so they do everything together basically.) We followed that with allowing him to have soft nuts much earlier too - cashews, pine nuts, walnuts. He'd see my daughter with them and wanted some too. One night he put a small piece of a cashew in his mouth and immediately started screaming and raking his tongue to get it out. We just thought he was tired and fussy, but realized quickly that he was starting to wheeze, his face was swelling and he had a rash on his face and hand. I immediately grabbed the benedyl and dosed him. His symptoms didn't progress any further and started slowing down. About an hour later I nursed him before putting him to bed and he started scratching all over. When I looked at him, he had developed a rash from head to toe. This was late on a weekend night so I hadn't called the ped since it was under control after the first incident. I called after the second since I couldn't do more benedryl. They said it sounded like he was fine since his breathing was under control, but that they would have had me take him to the hospital if I had called earlier. I had eaten a bite or two of the nuts when I gave them to him earlier in the evening and it went through my milk to give the second attack. He had a blood test the next day and it was neg for all nuts. Obviously this didn't make sense so we went for a skin test which was pos for pistachios and cashews. Our house quickly became a nut free zone and he couldn't nurse for 48 hours to let it get out of my system. He had had cashews before. Actually my ped said that you have to be exposed to it once before a reaction occurs because your body creates antibodies that first time and then attacks it the second time with the antibodies. We don't really have any severe allergies in our families either. My father-in-law periodically gets a swollen lip but hasn't been able to clearly associated it with any particular food or other item. Now my son cannot have any tree nuts (anything other then peanut), but he's not allergic to peanuts according to the blood and skin test. Before letting him eat peanuts freely, they want to do a "peanut challenge" in the dr office across from Children's hospital in case of an emergency. Basically they will feed him increasing amounts of peanut products for several hours and check for any sign of reaction. If that is clear, then he can eat peanut products.
Just watch closely and be sure to give him the peanut butter when you are there to watch him yourself the first several times. You don't want to have a sitter/etc have to handle an emergency. And don't assume that if he's ok the first time, he won't develop an allergy.