First off I'm sure you're just happy he's washing himself! But is there any offence taken that he may not want to use your towels or any discretionary reason that you don't want him turning up with a rucksack/bag with his towel, shower gel/body wash, body scrubber, facewash, fragrance etc.

I remember bringing my stuff to an appointment and while she wasn't offended, she look a bit puzzled and laughed it off as I went to the shower with all my gear in hand lol.