Oversupply or reflux??

Hi there. I am in need of guidance. I should start off with I am a very nervous first time mom of a beautiful 5 week old baby boy. I did day care for two years, and have babysat my whole life. However, I have no prior knowledge of breastfeeding. All I've ever had to know was had to add water to formula. I did not intend to breastfeed, but when my body started to produce colostrum at 22 weeks pregnant, I decided I would at least breastfeed in the hospital, and well, three days has now turned into 5 weeks. I can't believe I made it this far. I had such a hard time for the first two weeks. Sylar just wanted to sleep (so did I) and I had a hard time getting the both of us to stay awake and motivated for feedings. I did a lot of crying and I honestly dreaded the every 3 hour alarm because of how sore my nipples were. We conquered those issues, but now I have run into another what I would consider a minor speed bump. I tend to make things bigger than they are in my head, and its ten times worse when it pertains to Sylar so bear with me if this seems irrelevant. (I mentioned I was nervous right?) So, he eats every 3-4 hours during the day and at night he'll go about 4-6 hours between feedings. This has been approved by his pediatrician since he has been gaining weight exceptionally well. I let him choose when he wants to eat. He is a very aggressive eater, he gulps and does not stop anymore to take breaths like he did in the beginning. However 3 or 4 times a feeding, he will push his arms into my breast and throw his head back. I have now taken this as a hint to burp him. Sometimes it is difficult to burp him, other times he will burp immediately after sitting upright. If he does not burp, he will spit up. I put him back on the breast if he is still hungry. He will refuse if he is full. He has not taken more than one breast at a feeding since my milk came in and his feedings last typically 10-20 minutes. He never falls asleep at the breast anymore. I am worried that he is not getting enough hindmilk. I believe that my flow is stronger than he can handle which is why he gulps so much. I am not worried about the letdown because typically I letdown while taking my shirt off because I letdown anytime he whines. After a feeding is complete, he will almost always spit up. If he does not burp and is not kept upright for about 10 minutes, he will projectile vomit. He does not cry when spotting up, unless he's laying down and it comes out of his nose. Which by the way, he absolutely hates laying flat. All of this has been brought up to a pedatrician, she is not concerned because he has plenty of wet and dirty diapers and is growing efficiently. We do not have these issues after his dad feeds him a bottle. I would like to keep breastfeeding rather than bottle feed. Please help. Sorry for the novel, but I just wanted to provide as much info as possible. My questions are: How do I ensure that he is getting enough hindmilk? Is there anything I can try to make him more comfortable? Any advice would be greatly appreciated. Thank you.