Hen becoming a rooster??

[ILoveBabySilkies]

Most

How come his comb isn’t large though?

Most roosters have large combs, but sometimes they aren’t very large. I think that’s just the case with your cockerel.

 

[chrissynemetz]

The picture is definitely a cockerel. Sometimes it takes a little while for them to grow their comb in, and some boys take a long time to crow.

 

[sourland]

Comb appears to be a modified pea comb - they are generally not very large.

 

[CrazyCochin]

We have a rooster that we thought was a hen.

 

[50-45-1]

There are several different types of comb. Straight, rose, pea....etc.
Your rooster seems to have a type of rose comb.