Sounds like a display of dominance to me.
Also - could even be "mothering" of a sort. He's definitely biting - not licking? Our friendliest kitty to newcomers would pin little foster babies down - but he'd clean them, like a mum would do. If he's biting gently and the kitten doesn't mind, I'd just keep an eye on it and let them work it out. Maybe he's just a spaz at being a mommy.
I would consider getting your five-year old neutered now. Many vets like to wait until they're six months old. Ours always recommends neutering as soon as they start teething - which is usually in the fourth month. But to ensure any blossoming adult hormones won't be a problem in their relationship (or your home), I'd take care of that now.
Also, because your 5-month old will be teething (wanting to bite things), I'd also recommend getting a bunch of bendy straws and scattering them around the house. This way he's got what to chew on instead of the new kitten or your ankles.
The best way to discourage any cat from doing anything you don't want him to is to hiss at him and blow a short, sharp puff of air in the face, and walking away. (Don't want them to learn that doing bad things gets them attention!). Basically, you're talking to them in their own language.
If you're worried about the older fellow being too aggressive, do the same. Hiss loudly at him and immediately lean over and blow a short, sharp puff of air in his face. He'll be startled and let go of the little guy. If they're actually fighting, and little guy is struggling, obviously don't get your face in there. Get a can with coins in it to rattle. Loud noises - even clapping - will stop him.
Make sure you give your older, resident kitty extra attention. Because the baby will want to dominate any play, take your older kitty into a separate room, close the door, and give him a good 15 - 20 minutes of extra "alone" play time every day. This will help him destress.
I'd also take a towel, rub baby kitty all over with it, and at the end of your alone play time, put treats down on that towel that smells like baby kitty for your older boy to munch. This will help him associate the new kitty with good things.
Also - any time you see them interacting well with each other - playing "nice" or older kitty grooming baby kitty, praise him/them to high heaven.
The idea is to help resident kitty know he's completely loved and secure in that - and then to get him to associate the new kitty with good things - and to discourage aggressive behavior gently and in a "language" he can understand.
He will display dominance, and that's natural. He won't necessarily end up as alpha cat - but the two of them will have to work that out, and you can't decide for them. New kitty may try to display dominance by putting his paw on your older kitty's forehead - how he reacts to that I can't say. Could be with a hiss and a bat of his own back at little kitty's forehead. This would be normal.
With more than one cat, vertical space is very important. They often like to retreat to high places - and older kitty can easily jump and get away from little kitty bothering him. So take a look around and see what you can do to create vertical space for them. A cat tree or two is always a good thing - especially in a multi-cat household. Being able to get "up" to get away or to display dominance means he doesn't have to take it out physically on little kitty.
Even cats that love each other sometimes get in each other's faces, and a little bit of hissing, batting, or even growling at times is normal. However, watch the body language. If the ears are back, and/or the back is "ridged," and/or the tail is flat and fluffed - or flat or down and waving in a very "agitated" way - you may want to keep an empty can with coins in it to rattle. (Although when involved in intense play the tails may fluff - then you watch for whether the tails are up, or are really agitated). The loud noise will startle them out of whatever attack is happening or about to happen. But if tails are up, ears are up - it won't be too serious. Tails up is a good thing - kind of a kitty greeting, and if they're romping - but it looks like fighting to you - it's just play.
Hope this helps,