I am so sorry Pav and can well understand how upsetting this is esp as he is so young
. As Gill said, I would telephone RSPCA 1st thing say 7am and lie about how long he has been there. Make sure they understand how young he is.
If they don't help, phone a locksmith and explain that your cat is stuck in your garage and you have lost the key as you do not use it very often. Naturally you will need to make good the lock and leave a note on the padlock so the garage owner can contact you. One night won't hurt him. He may be young but 3.2kg is a big boy
Remember when our Millie was stuck way up high on on roof during terrible wind and rain all night long? She didn't even get the sniffles. She has never climbed up there since though and I'm sure MSG will learn a similar lesson about garages. You may well find he is not quite so insistent about going out for a while either.
I'll be thinking about you both overnight Pav
ETA This post was from me in a personal capacity of course and not with Purrs Moddy hat on