Whether he thought it was a boy or not, he should still have had the cat neutered. That was pretty irresponsible of your ex, as well as the fact he then left the cat with you. Unneutered males will roam for miles in search of a female in heat, they will get into fights with other males, and FIV is very common in unneutered cats, which can be caught easily through fighting and mating. Also, the cat population is getting out of control and they are suffering because of it. There are just too many cats and too little homes.
I've got 4 cats, and would have given this mum and her kitties a home. Well done for keeping her, and her kittens, until you could find a good home for them.