Not able to find dlc's from xbox 360 on a backwards compatable game on my xbox1

My son plays fallout on his xbox, previously on his 360 and now wants to play it on his xbox1 which the game is backwards compatible on. the problem is he is unable to get his xbox1 to find the dlc's he purchased on his xbox360 and it says that
it should know which dlc's he had purchased previously but it is not working correctly any help would be greatly appreciated. have tried to contact Microsoft but it seems there is no way to actually get to speak to a real person there(extremely frustrating)

Unfortunately it'**** and miss as to backwards compatibility. For the most part developers ported the game to the new consoles but DLC may have been integrated or it was not selling well so it was abandoned etc.

You double checked that they do show up on his 360 if he plays on there?

Talk them into playing a backward compatible 360 game from XBOX1 then they be in the backward compatibility mode. Then when playing tell them to send ya friend request again on 360 mode. Will work. I did that.

A 360 backwards compatible game on the xbox one and using the xbox one version of the game’s dlc with the backwards compatible one.

A cool feature that the xbox live marketplace is to allocate server space for ALL the original xbox DLC that is for games that is compatible backwards compatible, and DLC that is from all xbox titles that are free, but I guess some paid DLC can be included.

So here is the story. When you go to the marketplace, you won't be able to find the original xbox DLC, which is normal, because that isn't how it is unlocked or downloaded. To download it, simply insert the disc, and I believe that the 360 looks through the
internet and finds the backwards compatibility file, so along with that, it also downloads all the DLC there is for that specific game.

Now that would be a cool feature.

Before I continue, couldn't you even run original xbox games on a 360 slim? And am I right about how an original xbox game is run on the 360? I remember for people who don't have internet connection to their 360's, they had to download some file and put it
on a blank disc, insert that disc to the 360 and move the file to the 360 hard drive, and that is how the original xbox game can be played on the 360.