If it's truly an acorn, I'm afraid that penetrative sex might have to be off the menu for him.
I had a client back in my agency days (the guy who smoked crack, for anyone who's heard me ranting about the insanity of that particular situation

) who had a tiny acorn which was obviously even more shrunken due to his drug use. No condom would possibly stay on - so neither I nor any of the other dozen girls he would book allowed him to do any penetration; it was only handjobs (even blowjobs were tricky cos he had a bit of a tummy and would never lie down cos he was too busy running around the huge apartment checking behind the curtains for aliens or police or something). I mean, of course someone might have allowed him to do unprotected penetration when nobody else was looking, but that's not the point here - basically, he was just too small for condoms.
It reminds me of the issue of clients with erectile dysfunction who insist upon trying to put their limp penis inside you; you end up frantically trying to hold the condom in place while he tries to push his willy in with his fingers. It's just ridiculous and, considering the vast quantity of alternative pleasures available to them, I think a client who insists upon penetration despite the fact that his penis is not large and/or hard enough to safely wear a condom is not a good client.
Perhaps your client is simply unaware of the risks he's pushing you into taking? Try talking to him, maybe, and suggesting oral and hand stuff instead. I'm sure he doesn't enjoy freaking the hell out of you with that slippy condom issue really.
