I'd say that your adult son's boss (because he really is his boss, and not primarily his friend, if he hired him) is not necessarily too old to be his friend. Sometimes it happens in the workplace that the boss will organize more social events with employees, so that is not necessarily unusual.
But this guy certainly sounds questionable in your mind, and maybe in your son's as well. It's tough with the job thing, and it might be hard to find a new job, so it is a dilemma. But I would say that anything that makes you too uncomfortable is not worth it - more often than not, trusting your instincts serves you well.
Btw, I would say that your son is legally an adult, but intellectually, research has found that people do not fully mature in brain development until around age 25, so he is still in need of good advice from mom and dad, especially if he is in a vulnerable place of feeling obligated to someone because they hold the key to employment.
Best wishes.