It’s not so much that servers are ‘safe’ or ‘unsafe’ – there’s not much opportunity for real-world interaction on them.
The key things for me are: Make sure he plays it in a family area, rather than up in his room – and that he turns ‘chat’ off and keeps it off. There is a lot of swearing and abuse from randoms on chat.
Also, on servers, people do gang up on other people and nick their stuff, hassle them etc, so you need to keep an eye out and just teach him to find another server in those circumstances – as you say, there are plenty around.
Drac’s suggestion is a good one if he has some schoolfriends he wants to play with.