In Ottawa we have things like First Words that will do an assessment of speech and offer suggestions. At the same time they will identify if they think there is more to the issues than just delayed speech. Coming from someone outside the situation often holds more weight with parents. The doctor wouldn't have experienced a lot of the issues you mentioned in a normal visit - especially anything related to food.

For the potty training it is often one of those things you teach the same as teaching them to use any toy. Start by routinely taking him and putting him on the potty - use the regular toilet and not a little potty since he doesnt' adjust to change best to start with a "real" toilet. If he goes fine and if not fine. If he doesn't go try him again in 20 minutes. If he goes try him again in about 45-60 minutes. It will just come with time to teach him the words to use but for several months it will be you initiating the potty trips and not him.