L.B.
I am an RN and in all the hospitals that I worked the speech therapist are the swallowing experts. If a patient has trouble swallowing the speech therapist is called and he/she does an evaluation.
The speech therapist will feed your son food of different consistency and textures and monitor it from the minute it goes in his mouth, while he chews and then swallows. She will use a special camera to watch the food move down the esophagus. It is called an FEV evaluation and it is painless. The speech therapist should beable to determine the cause of the dysphasia. It could be many things, perhaps muscular? If it is muscular the therapist can help him strengthen his swallowing muscles. If it is something else, she will be the expert to know what intervention works best for your son. It would be irresponsible of me to guess what could cause your son's dysphasia without more info and further testing.
Dysphasia is just a word that means difficulty swallowing, it does not give a cause or reason for your son's difficulty swallowing.
I suggest that you call your pediatrician and tell him that you want your son evaluated by a speech therapist ASAP. Why wait??? Trust your instinct, if you think it is more than just being a "picky eater" it probably is. If the test come back normal, then you know and you have peace of mind. If something is wrong - it can be managed and your son can start enjoying his food.
Good Luck