Hi Scarlett,
As other good folk have already mentioned, he's old enough to know, and do, what he likes.
If he has no luck with women, he has to survive so maybe the only way to get by is what you suspect he might be doing.
Have you suggested he could join a singles night club......plenty about if you look for them....or a dating agency?
In regards him going to seek out a prositute, he must know the safety aspect and need to carry out safe sex, and hopefully he will, so no harm done.
I dont know your personal feelings or situation, but if you are single and you feel anything at all for him, more than a friend, let him know.
Be his friend and not his jury. :0)