If he still has his testicles, he is NOT fixed. You don't need to bother inspecting his penis. When cats are neutered, the testicles are removed.
Cats are induced ovulators, which means that the female in heat ovulates when the male inserts his penis (that's what the spines on the penis are for - to trigger ovulation). Gestation is approximately 60 days - so 2 months from the time he was mounting them, you are highly likely to have your cat population increase dramatically.
An experience vet or cat breeder may be able to tell as early as 3-4 weeks that a cat is pregnant, but otherwise it may be more like 6 weeks or longer before you can really tell. And sometimes not even then. But if the girls haven't gone back into heat, there is a good chance they are pregnant. Especially from the behavior you are describing on the one.
At this point, depending on how far along they are, you have 2 choices. You can either let them have this batch of kittens, find homes for them all, and then get the moms spayed as soon as possible after. If you chose this option, you will want to get the male neutered ASAP - ideally before the kittens are born. Because female cats can come back in heat as early as 2 weeks after their kittens are born - while they are still nursing the first litter! If you wait to neuter him, you may wind up with another batch of kittens.
The second option is to go ahead and get them spayed now, whether or not they are already pregnant. Most vets will spay a pregnant cat, though some have different cut offs as far as how late in the pregnancy they will do it. I know some people have a problem with spaying a pregnant animal, associating it with abortion. However, the reality is that healthy adoptable cats and kittens are euthanized by the millions every year, and even if you manage to place every kitten your cats produce, that takes away a potential home for a cat or kitten that is already out there. As much as I hate doing it, I'd rather spay a pregnant cat than euthanize 8 or 10 week old kittens that couldn't find homes.
Hopefully this is a non-issue and he wasn't able to impregnate either of your girls. Unfortunately, I'd be willing to bet that at least the 1, if not both, are pregnant, so would recommend having them checked out as soon as possible, and getting the boy neutered even sooner.
I'm sorry if that isn't the news you were wanting to hear. But at least you don't have to try to inspect a cat penis.