The fun only begins when the inner sphincter opens.
Keys to enjoying being fucked by a 8.5+ dick: lots of lube generously applied as he slowly enters; might be easier kneeling rather than on your back; as he pushes in, gently push out, as though taking a dump; some lube injected before he enters will help a lot; poppers offer a lot of help in this type of thing, they open those areas really efficiently -- but poppers are regarded as somewhat contraversial in some quarters, use your own judgement; a few glasses of wine or a joint will make this inner penetration easier.
One of the key things is whether the top actually knows and understands where he is aiming his dick. I think a lot of bigger guys don't really know how the ass is configured and where they might be able to put that thing if they were more aware.
It feels really awesome when the head finds the rectal valve (inner sphincter) and gently tickles the opening. This takes time to find, for some, but when he does his bottom will likely make a welcoming noise, like "ahhhh, oh yeah!" At this point he has to be patient especially if the bottom is not used to having that area penetrated. Lots of lube, gentle massage with the head of his dick, a little poppers (or a joint or drink beforehand) and that valve will open just enough for him to begin opening it with the head of his dick. At this point it will begin to tickle in a really, really good way. He still needs to be patient, though, as although the tickling feels good the stretch of the valve can be a little achey. But as the cock head hits the nerve endings just beyond the rim of the rectal valve it is highly stimulating and will be intense but good; the stimulation itself will begin to make the bottom's ass WANT to open. It becomes such a good itch that your inner opening just keeps wanting to feel that sensation. At this point there is no more hitting a wall. He can begin to go balls deep with a great feeling for both. As the scene becomes more relaxed and the holes more open, keep adding lube. Now he can begin -- in time -- to take his dick father out and gently slide back "home" with much more ease. This feels really, really good because he is hitting all the nerves around the anus, the prostate, the great areas in the rectum and finally the repeated stimulation of the rectal valve and beyond which, combined, can produce a really mind blowing symphony of sensations. It is when you have experienced this that bigger dicks become the definite desire rather than a frightening possibility.
Getting hot just thinking about those bigger dicks!!
Hope this helps!