Why is my baby sick all the time?

my 4 month old son is sick constantly throughout the day. he has been like this since he was born and now seems to be getting worse! when i feed him his bottle i wind him halfway through and after and he starts screaming because he doesn't like it. i also keep him upright for at least half and hour afterwards to Held the feed go down. Then all through until the next feed he often burps and a load of sick comes up sometimes its often projectile.Although he is always sick he is thriving and putting on weight. ive spoke to health prof and they have all told me theres nothing to worry about and he is just a sicky baby and he will grow out of it. its just starting to get me down so much now! has anyone else had the same problem?