I bought my son a Remington 1100 20 ga several years ago. It does not have choke tubes so I guess it wasn't made with them. He has talked about either buying another barrel or having his threaded and adding tubes. I don't know anything about that so I was wondering if anyone had any advice on which route to go.

Without measuring it, I can't say for sure, but if you want to use factory Rem-Coke tubes, you'd probably need to get a new barrel.
It's been my experience in the past, the older non Rem-Choke barrels don't have a large enough OD to be retro fitted with anything other then Tru-Choke pattern tubes.

I believe this gun was made in 1978. That's where my research sent me. The barrels that I looked at were around 350.00

Yeah it would definitely be less expensive to ream and thread what you have.
You just may not get to use a factory choke tube pattern,