ENT or Immunologist?

Hello, I need to take the next step for my son and his allergies. He has battled problems since he was very young (he is 15 now) and we have been back and forth with the GP. Over the years, he has been constantly on antibiotics, allergy meds, nasal sprays... etc. however he never seems to get better 100%. Some of these things help but not enough . He has been a mouth breather since I can remember because he cannot breath through his nose very well. He is either stopped up or he feels like he doesn't get enough air just breathing through his nose. He also constantly goes through tissues from a runny nose and sneezing.

We did go to an ENT a few years ago although I wasn't satisfied with that visit. I don't think the doctor ever spoke to my son or look him in the eye nor did it seem like his exam was very thorough. Since then we have been back and forth with the GP and I've had it. Do you think that going to another ENT is my best bet or should I just go for the immunologist? I'm just afraid that the ENT would be more likely to just prescribe something and send us on our way vs an immunologist who will get down to what the problem really is because we have been thrown so many theories (from allergies to small nasal cavity (which we have already had Xray'd)).

Thanks in advance for any advice.