Hi OP, I’m a lone parent and have young adult DSs.
I wouldn’t talk to him about masturbating. It’s a perfectly natural thing to do. Both young men and young women. But also a private thing, which it’d be excruciating for your parents to talk to you about. It’s important to talk about sex as you go along: consent, safety, contraception, porn etc. But I’d never discuss masturbation.
If he’s leaving tissues lying around his room, don’t mention them specifically, but get him into the routine of tidying his room at least every weekend. Once he’s up on a Saturday. Bed made and/or bedding changed. Dirty clothes in the wash basket. Clean clothes put away. Any rubbish, including tissues, in the bin and empty his bin into the bin outside. Any glasses, cups etc downstairs and washed. School stuff organised. Surfaces wipes. Carpet hoovered.
This is what adults have to learn to do. Men and women. And he’s a young adult. You’re teaching him to look after himself for when he lives independently.
With regards the towel, mine have hooded towelling bathrobes they use for showers and keep on the back of their bedroom door. No need for family towels other than a hand towel. But if he uses towels, then buy two for him in a different colour to the rest of the family, perhaps to match the colour scheme in his room? Dark grey is good for a young man. He keeps his in his bedroom on a hook. Each week, or each time it’s dirty, he puts it in the laundry and uses the clean one, stored in his room. A couple of his own hand towels may also be useful for the masturbation. Again, fresh ones stored in his room.
My DSs also have a box of tissues in their room to stop them endlessly taking toilet roll: not just to masturbate but for colds, as serviettes for snacks etc. He could also have a pack of face wipes or baby wipes, which you buy and leave in his room with the tissues. If he asks what they’re for, say you thought they may be handy for a quick freshen up, e.g. sweaty pits. He may get the message.
Finally, you may already do this, but I never went in my DSs rooms without knocking once they were teens. Partly to avoid walking in on them masturbating. But partly because young adults should be afforded privacy. I didn’t even go in while they were out, other than to perhaps scoop up any dirty crockery when tidying up. But with adult privacy comes adult responsibility. Initially (around 14-15) their rooms got like a landfill site. But once I’d got them into the routine of a weekly clean and tidy, they started to take pride in their rooms and now they do it of their own accord. Obvs still messy, but not with rubbish, dust etc.
He should choose a couple of household chores he’s responsible for every week: taking out the rubbish, hoovering the stairs, mopping/hoovering the hall, sorting clean laundry etc. Plus, washing up/seeing to dishwasher two evenings a week. As should your DDs. You’re now pretty much a household of adult sharers. So the household tasks should be shared. Mine (early 20s/late teens) cook dinner at least once a week now and really enjoy it.