strange dog behaviour?

I have two female dogs, both are about 2 years old and spayed. I look after kids during the day and one of my kids got a puppy a few months ago. I told Mom she could bring the puppy some day for a play date when she brings her son. so they could socialize him before it's too late. Today she brought him, he is 7 months old.

This is the oddest dog I have ever seen. He is constantly trying to mount my girls, And yeah he is a boy and yeah he isn't fixed but he won't do anything else, I watch him and he has a very aggressive looking head and shoulder when he approaches them. They don't like him at all and snap, growl show teeth and move away. He won't stop bothering them, seriously I had to put him in his kennel so my girls could just relax for a few minutes. I'm used to the play bowing and that sort of thing but he doesn't do that just tries to sniff their bottoms and mount.

When he is walking around he is very friendly and social, (like I said way too social) tail wagging and even good with the kids. I figured I'd put the leash on him to keep him from mounting the girls and he is a totally different dog, tail tucked between his legs, laying at my feet. Very withdrawn and I would say scared.

My girls never act like this, not the mounting or the snapping or the growling, and I get that this is their first visit but he is just a puppy, a weird one but a puppy none the less. My girls the minute I get the leash they are not excited but very happy and tails wagging and they are happy, no other word for it but happy. Not a bit of cowering even though one of my girls knows that boots are soon to come and doesn't like them but even then she doesn't cower.

So I probably won't do this again if it was summer I might consider it but it's -20 and too cold to go out for a walk with the dogs AND the kids to get his energy down.

So my question is how should I be handling this, do I leave him to the mounting? Let my girls snap? I've been allowing it to a certain point and then I just step in between them and don't let him close to them, just moving so he can't approach them. I've been doing the psst sounds when he gets really aggressive shoulders and tapping him on his neck.

All three dogs are less than 10 pounds, my one is 10, the other is 7 and he might be 10lb's soaking wet but that's it.

And should I put a leash on him even though he is scared? I'm not dragging him with it, just leaving it hang loose so he doesn't feel pressured.

Any suggestions.

Thanks