The consequence would be he stops being a very functional human. Having all these things will make for a very problematic disability.
a thick exoskeleton that is a lot like that of a mantis shrimp (to make him bullet-proof)
Bullets probably won't hurt him but larger calibers will probably still do. Although I think he would have problems moving around with a heavy shell like that. Also shells don't protect against shock so internal damage will happen even if the shell if undamaged.
very small holes all over his body to use to breathe instead of a nose (to make it hard to cut off his oxygen)
This kind of breathing is likely less efficient than just having a central system like the nose and lungs. Arthropods have a problem growing very large in real life and this is one of the reason why they might be limited. Back when the Earth was more oxygenated and there was less competition from other animals was when large arthropods existed.
have a lot of stem cells stored in his bone marrow (to quickly repair damage)
have the DNA repair mechanisms of Deinococcus radiodurans (to protect him from radiation damage)
Bacterial DNA repair would probably not work very well in humans. From what I am reading, Deinococcus Radioduran keeps 2 copies of DNA and have mechanisms to repair any damages within hours but it has a much smaller genome compared to a human and it's repairs are still susceptible to failure like during normal cell operations. What I imagine happening would be cancer cells developing from his cells being extremely resistant to chemotherapy. Can you imagine self repairing cancer cells that is resistant to chemo? The stem cells might be nice to have tho but I don't know if it will help this guy with much.
At the end of the day, I don't see this person functioning very well as a human. I think it would be very hard for this person to do anything or go anywhere, not only would it be hard to move in his large shell but it would be hard for him to use any vehicles. It might even cost a lot of money to keep him alive because of this disability. Not being able to breathe well and having such an unusual body would require specialized medical attention, I imagine he would need a very highly oxygenated room as well as a specialize bed.
As for food. I guess the hospital might need to make sure he gets more protein than the average person as DNA repairs going on the background might need more protein. Maybe he would need more other nutriments to supplement his shell as well. I'm not sure what kind of exercise plan they can give him because moving can cause server internal bruising and using too much energy with the limited oxygen he can access to can cause serious problems. I guess keeping him completely still might be best for him. In this case it would probably mean limiting his calorie intake as well.