The easiest approach is similar to the oxygen tank approach, keep him eating normal human food, simply figure out a way to provide it for him in the alien enviroment. There is no reason a human can't take off an oxygen mask long enough to take a bite of a sandwich and put it on for instance. Any advanced civilization will likely be able to grow something compatible with his dietary needs much easier then changing those needs. If this is too low-tech though you can provide food/nutrients without going through the mouth.
You could assume a situation where they can't just remove an oxygen mask though. Perhaps the enviroment they are in is so hostile that being exposed to it at all is dangerous, think space suits that astronauts have. If someone has to wear that he may not be able to take it off to eat his sandwich.
The next less invasive approach would be to have their oxygen mask include a food dispensing item. Imagine one tube for water, one for food-paste. This approach would allow them to choose when they eat, and even eat different 'food'. Depending on level of comfort desired this could be made somewhat convenient.
If this approach was too complicated or not desirable enough the next logical approach would be a feeding tube which would provide nutrients constantly. The same 'tank' that provides oxygen could have a second smaller tank to provide nutrients. However, feeding tubes aren't entirely comfortable to have in, so aliens would probably prefer the less invasive feeding 'tube' that one can suck on when their hungry. However, with a little technological skill it would likely be possible to create a feeding tube that is less invasive then the ones we use, likely one that would not go through the throat/nostrils but through the abdomen.
However, there are other concerns with such a potential approach. This implies a life lived in a space suit which would NOT be very comfortable. Your body needs to breath, to bath, to be itched etc etc. People in hospital beds get all kids of sores simply from not being allowed to stand up, people with casts itch like crazy. If you were confined to a suit like this non-stop it would NOT be fun. I imagine there would be a severe psychological impact of being stuck in a suit like that as well, a sort of claustrophobia.
If your aliens truly wished to make someone comfortable they would likely provide a small controlled enviroment that emulated human/earth enviroment. A room where you can take off your suit, breath naturally, take a shower, and eat normal food. Thus all these other options would be more of a 'I can't get home right now and need food on the fly' approach rather then a long term one.