I have seen that happen once in a great while when flying with a full crop but if it is happening every time he flys I would have him examined by an avian vet. In may be that the crop is slow in emptying and the exercise of flying triggers the vomiting, does he vomit at any other times? Sometimes there can be an infection and we don't even know it. I recently took 2 of mine in for their annual physical and upon the physical exam the vet said one of the birds crop did not feel right so a crop swab was done and sure enough there was an infection going on and she is now on antibiotics. She did not exhibit any outward physical symptoms that would indicate she had this infection, so sometimes you just never know. Had she not gone in for her physical, eventually she would have become ill enough that there would have been noticeable symptoms.