HELP 3 year old and food allergy question?

[Soooo Sleepy]

I'm getting fish oil for my 3 year old son who has cerebral palsy. He has "food sensitivities". How common is fish allergy?

He will have to take the liquid because he can't swallow pills.

Also, which one would be more common: lemon/lime allergry or mango allergy? Some of the fish oils are flavored with these.

And of course I'm going to ask the doctor about all this, but sometimes it's good to get opinions from people who have more experience (ie people who actually have food allergies).