I'm not sure whether I'm mansplaining here or gaysplaining. Those who are more adept at noting these things will no doubt let me know.
OP, it seems you have two issues here: firstly that you've found out that your DS is gay, the second that he's messaging a guy five years older than him.
Issue 1: ye gods, don't mention it before he does. It is a massive thing, coming out to your parents, and pre-empting it is just awful. Drop hints and emphasise how liberal and accepting you are, by all means, but don't ask him outright if he's gay. That's his to tell people as and when he feels happy to do so. My parents were among the last people that I told, because their reactions meant so much to me. (My mother, who was a florist and involved in AmDram told me not to be so daft, as I'd "make a crap gay", while my homophobic father said "I'm happy that you've finally told me". Horses for courses, I guess.) Ultimately, it's something you need to "let him have", as it is formative as a homosexual man.
Issue 2: Eeeh. For all those that say "if it were a fifteen year old girl messaging a twenty year old man...", I'll point out that for most gay teenagers, there's precious little out there. Your son might be the only guy in his year (as far as he knows) to be gay: where's he going to find somebody to explore this with? Is he just going to have to wait a couple of years until he gets to university?
On the other hand, if I were a parent, I would be less than impressed. For example, when I came out at 16, my first proper "gay" boyfriend (as opposed to any "experimentation") was 26. I would have loved to have had a boyfriend of my own age, but he was pretty much the only other gay single guy I knew. We went out for about six weeks until he dumped me. A few years later I found out that my mum knew his mum and had had a word, pointing out that nobbing a sixteen year old boy was a bit noncey, and so he dropped me.
I didn't feel taken advantage of, far from it. However, in OP's case I would monitor the situation closely: if her DS is sixteen soon, then it's legal soon. However, that doesn't necessarily make it right, but nor does it make it wrong. Keep an eye: is he happy, and is he being taken advantage of? Looking back, I "took advantage" of my much older boyfriend as an "in" to gay society- he never took advantage of me. As long as your DS is the one in control, then maybe this is something that he needs as a stepping stone? I know that's what my boyfriend was to me at the time.
OP: in summary, don't say a word, just keep an eye and be there. That's what my DM did for me, and my god but do I thank her for it.