As long as it is used exclusively by him and he cleans it every time to prevent the spread of any possible bacteria, he should be OK.
Well, there is one -- if he uses it hard enough to cause cracking or bleeding, it makes a nice little home for bacteria and viruses. It's why AIDS spread so quickly, if he's using it on his anus. That's a very delicate area. So even if he cleans it, when he has sex with someone else, it does make it more likely if it's being used roughly that he'll be vulnerable to whatever they have.
None..as long as he cleans it if he uses it on someone else.