Does he have any conditions we should know about? Diabetes, anemia, anything? Would you say his diet is well rounded? Imbalance of proper nutritioning could lead to the problems you specified.
For the moment he needs to take it very slow and easy obviously, he needs to find his own limit, understanding what is too much and what is too little. Don't have him set goals like "I'll do this 30 times and then rest" or "I'll do this much weight", it will probably lead to overexertion and cause those exercise induced nausea. Whatever he can do where he feels like he got a bit accomplished, but not deadbeat on the floor because he did one too many sets and throwing up. What are your trainers saying?
And yes, you should probably check to see if he is holding his breath, it is a bad habit that could lead to feeling lightheaded and possible blood vessel ruptures.