Unfortunately, a mouth can handle only so much oral sex. (it's true). It's the same for big and small penises alike. I can understand the difficulty in trying to orally pleasure a man with a large penis. Using the tonge is definitely important. You can also make sure to use a lot of spit, and use it as lube. You can take breaks to relax your mouth by stroking him. It feels great, and it allows for a lot of eye contact, which, in my opinion, is really sexy.
use your hand when i get tired i just flick my tongue around the head and use my hand to continue on him ...it seems to work and im glad because there's no way i could handle doing that as long as he needs
With My member being on the large size, six and a half inches circumfrence, It is very very infrequently that I have been able to have any success with oral. Some "paid proffessionals" have managed but most women cannot open wide enough. My present long time G/F does a creditable job of sucking and licking which is quite exciting and stimulating but with no "in-the-mouth" bit I do not ejaculate but thats OK. There is great pleasure to be had without ejaculating. Just enjoy what you can do but don't suffer the discomfort of trying to get it in your mouth. You are probably not the first he has tried it with so he should understand accept he is too large.