Hi Oscar,
He can get his first shots between 8-12 weeks. To the best of my knowledge, no cat has had stunted growth. Since you’re moving to another country, make sure your kits have all their needed shots. It’s a real pain if you get there and they have to be quarantined until they get the shots. The UK is pretty strict about this requirement.
Kittens/Cats will probably be lethargic after their shots. That is purrfectly normal.
Some cats are allergic to the rabies vaccine and may have a serious reaction to it, known as anaphylactic shock. Signs of shock include extreme lethargy, reduced appetite, collapse, trouble breathing, vomiting and seizures. These effects usually occur within an hour of the vaccination, according to the Cornell University College of Veterinary Medicine. You must bring your kitty in for emergency veterinary care if you notice these symptoms, as an allergic reaction can sometimes be fatal.
Keep an eye on them but they should do just fine.
I hope you can find new homes for your kittens.
PG