ferrall cat

is it ok to put him in garage overnight?

Welcome to The Cat Site.

Without knowing the background (details, please --how feral?  why in the garage--recent spay/neuter, etc. ) ), yes, it is okay to put the cat in the garage with a couple of caveats, suggestions:

Keep him in a carrier/trap so he can't get into anything he shouldn't get into, like a car or hidey-hole you can't extricate him from.  Cover the carrier/trap with a blanket, they feel safer that way. 

Is your garage heated?  Though no doubt it's warmer in the garage than outside (where do you live).  You could plug in an electric blanket or put some straw in a box/Tupperware container.  The cat will use that as a safe place to go to, and the straw is a great insulator and blankets are good too in this case.

If he has full run of the garage, simply put out some food and water and a litter box.  And the box/Tupperware plastic container. 

I agree with confining him.  Unless it is completely empty, with no shelves or cabinets to hide behind, he should be confined in a smaller area.  A crate is ideal.  You can put a litter box and bed in it.  It may also help with socialization, as he will begin to see you as the nice person who feeds him and keeps him warm.

Whatever you do, have him neutered as soon as you can.

I have a friend who's entire garage is her feral cats' kingdom.  There is a cat door they use but inside, there are beds, litter boxes, a separate feeding area and shelves, shelves, shelves!