My son has a swollen tounge and blisters inside his mouth any idea?

Yes: Need more history. Could be he burned his tongue on something hot. Could be a reaction to acidic food. More likely it is a viral infection. He needs to be seen and evaluated. How swollen is the tongue? Any trouble swallowing? Drooling at all? These are things that are best addressed in a visit.

Many : But it is impossible to make a diagnosis in this forum. Your son needs a professional evaluation asap if his tongue is creating difficultly swallowing or with breathing. Get to an er asap of those symptom are present.