Ok. I may be comming out of left field here.
(But it kinda sounds like me)
I am about the same place in the relationship, over the MUST have it ALL the time (he even had me convinced that there were studies that sex was PROVEN to alleviate the symptoms of .....fill in the blank)(and he's not even CLOSE to being a teenager!)whatever....
and have settled into real life. I get urges and sometimes he just isn't there. (there are ways of maybe changing his mind, but at the first rebuff, remember, NO means NO.)
And there have been many times when I just haven't felt like it but BOING!!!!
You have said that once you get going, you are OK, and he is getting frustrated.
Is he waiting for you to initiate? or do you just not respond to his advances?
I'm gonna go with the not responding so he gets frustrated.
(always knowing that it is still your decision)
RELAX! Even if it is NOT earth-shattering, it can still be fulfilling.
Introduce a lubricant into your life.
Honey knows that I am going through physical changes, and has come to terms with the fact that he cannot always INSTANTLY spur me to self-lubricate
(Gosh, when I initiate, I can't always self-lubricate!

)
Soooo I have taught him how to apply the KY, or whatever .
He will not be happy about it at first, but as soon as he realises the benefits, he will be OK. Relax and tell him where to touch you with the lotion. Make sure to over do rather than skimp. (We always have a towel standing by for after, but it could be used before too!)
Give it a go.
I know Honey and me are NEVER on the same schedule, and sometimes when he comes at me with that big grin and flips off the covers, I KNOW what he wants, and I kinda roll my eyes, point at the bed-side table and say "show me, baby"
Sorry, the rest, you have to make up yourselves!