Our son is asking our advice for one of these. From what I've read it seems that most TV/DVD combinations will not play a Blu-ray DVD but not sure why.

I tend to stay away from combinations. My own advice would be to buy them separately.

Any advice appreciated.

Most blue ray players now also play DVDs. If it only plays DVDs I would avoid it. Our newest smart TV (Samsung 55in curved screen) communicates with our Sony blu ray/DVD player and automatically switches to DVD/blu ray and starts the movie when a disc is inserted into the player. I think you are right, buying them separately is the way to go. DVD/blu Ray players are pretty inexpensive now.

We had a TV/DVD player a few years ago. Most people have told me the DVD player would go bad first then you would have to buy a DVD player. In our case the TV want bad first. From that point forward we only buy non combo TVs.

Two rapidly evolving technologies - discs (DVD->BD->3D->4K Ultra->?) and TV screen resolutions and technologies will go obsolete even faster when you lock into both at the same time.

With separates, you can keep the one that's still good and upgrade the obsolete one, but with a combo--you throw out the baby with the bath. As 71 TRUCK points out above, this aspect is also true for unit failure as well as obsolescence.

The other aspect is that discs may go away over time as boxes connect to networks for their source materials and internet speeds support high quality streaming.

How big is the screen, cost, why one with a built-in at all?
Optical media is becoming obsolete so why get it?
Get one with a built in VHS instead.

He must be looking at a old, second-hand tv?

I wouldn't do it unless it was super cheap and I was on a really tight budget. My DVD collection has been collecting dust for almost 2 years now, as I use streaming and OTA for 99.9 of my TV viewing. When I do want to play a DVD, I have on old PS3 I can use for that. My point being, it seems like there probably better options, unless it's all that's in his budget.