Why is my nueterted male cat hissing and being agressive towards female cat and their kittens?

Were having issues with my 1 year old male siamese cat who was super affectionate & sweet he just recently got nuetered while my other female cat was pregnant they are both indoor cats and have been together since hey were kittens. He used to be very lovable and always wanted to be around us. Ever since my cat had the kittens he turned really mean he acts completely different. Very aggressive towards mama cat hisses and growls at her if she walks by him or just anytime. He's been hissing at the kittens too. But then sometime he just watches and ignores them. Or cleans them and then next min he's being mean. I'm very worried about him. He never wants to be around us anymore and its just getting worse. I miss the old him but I'm afraid if this continues were going to have to relocate him to another home. Oh n btw my cat had 3 kittens 2 females and 1 male. Idk if that's key maybe he's jealous or the the neutering??? Idk someone please HELP!!!! Ps: as soon as we can we plan on getting mama cat spayed. No rude comments! Thanks for ur time!!