We successfully dined at Ohana with pn/tn and shellfish allergies. My daughter is contact sensitive to peanuts, but she didn't have any problems. Make sure you note the allergies on your ADRs. At the restaurant, remind again at check-in and alert the server if he/she doesn't mention it. A chef will come to your table and discuss the menu and how your son can be accomodated. In our case, we didn't get the peanut sauce. They didn't have the noodles when we went, which I believe contain peanut butter. If you're not comfortable having them on the table, ask for an alternative side dish. My kids were also offered options from the Kona kids menu. The bread pudding wasn't safe, so the chef brought out giant bowls of ice cream for the kids instead.
Have the allergies noted on all ADRs at Disney. There are lots of things with cross-contamination issues and hidden allergens. The chefs do a great job keeping people safe as long as they know about it. Also, if you eat at CS, let the order taker know, so ingredients can be checked.