depends on a lot of parameters. What do they all talk about? Nutrition and lunch?
If they ask personal questions about him does he shut them down? Do they ask personal questions about themselves? If so does he direct them to the school counselor or their parents?
Does he allow touching to go on between him and the girls? Not a hug because this might be natural but lingering hugs?
Have you considered asking him how he thinks it looks to others? Not in an accusatory way just how he might not be aware that it could be misunderstood.
The point is don't ruin his life because you think he might be doing something. Make damn sure. All I can say is things sometimes are not what they appear to be.
If your suspicions are well founded, go to the school counselor and ask if you can remain annonymous, and that you could be totally wrong it's just you thought it strange.