I learned to manage it the first three days, so on the last day before I had to come home, I let him pound me hard.
It sounds like you were finding ways to cope before this pounding occurred. Therefore, the most obvious answer seems to be no more pounding.
He may be disappointed by this, but it sounds like your body just can't take his full length. That is something he needs to get used to if he wants to be with you.
In terms of accommodating his girth, yes, foreplay, good lubrication, and going cautiously are a must.
The first couple of minutes after my thick-dicked husband enters me are
always painful. His length can get me at first, too, but luckily I can accommodate that as my vagina relaxes and tents (he is not nearly as long as your guy).
The good thing is that after the first few minutes, if I can just hang in there, it gets a lot better -- even starts to feel very pleasurable, almost overwhelmingly so. That makes it all worth it.
When we first met (we were long-distance too) we were together for a week in his home town and later for a week at my place. Both times I let him fuck me silly all week and I was beyond sore...but I didn't worry too much because I knew soon I would be without him and I would have all the time in the world to recover.
Once we got together permanently, though, there was no way we could continue at that pace. We eventually learned that even with lube and everything else, I need at least a day to recover between fuckings. We simply cannot have vaginal sex every day, and that is just a limitation of my body (in conjunction with his size) that we have to accept.
But there are plenty of things to do on the off days -- no vaginal sex definitely does not mean no sex!