Neutering might help. Right now he's definitely trying to establish a
pecking order, with himself as the alpha cat.

When he pins the younger kitten down, is the younger one on his back at
the time? If so, it means he's not taking much guff from the older one.
Lying on the back is a cat's best defensive posture, because that way he
has all his natural defenses immediately at his disposal: teeth, claws,
feet, legs.

Hi Ellie

Get the older one down to the Vet a.s.a.p. and have him de-sexed!   Do it
NOW!!!  Don't wait any longer!..... once those hormones kick in and certain
types of behaviour become established it is so much harder to break the
habits that have been formed.  Hopefully that will curb some of the dominant
behaviour, although the cat may just have a more dominant personality.

Cats (and dogs) start their reproductive lives at a very young age, and
throughout their lifetime can potentially deliver many litters of offspring.
(It has been mathematically calculated that in just 7 years, one female cat
and her young can in theory produce as many as 420,000 cats!!!)
Cats can get pregnant by as young as 5 months of age, so you should take
your young cat (male or female) to be de-sexed by the time it is 5 months
old.

~Roberta~

If you like, I'll lend you Junie....She's the bitchiest female cat I've ever
had....She has the two males under her thumb, and one of them is an unfixed
feral cat! Junie rules the roost, and won't take any gaff from the other
three cats........

Five months old, is plenty old enough to be neutered.  You are right,
he's practicing being the dominant house ruler.  Of course you play
with both of them a lot?  you know that your playing with them
distracts them from each other and lets them know who the REAL
dominant "Cat" in the household is [you].  -- summerrose