He is allowed to do the questions in any order, but I would advise very strongly indeed against that approach. As Guest55 has said, time-wise it is a very high-risk strategy, and she is correct (of course) in saying that the question types are not labelled in the real test.
Usually the only question that should be left to the end is Type Z, and he should circle on the question sheet any questions that he has left unanswered because he was unable to solve them, so that he can revisit those at the end.
If he were to do that for all the questions he doesn't like, he would be wasting a great deal of time moving around both the question and answer sheets. It also means that by the end of the test he has a big pile of nasty question types waiting for him, which is hardly conducive to motivation!
A better idea would be to insist that he does them in the order that they appear and, once he has got through a type that he doesn't like, think to himself "phew, I made it over that hill"!
I am finding with my son that, when he says "I hate/I'm rubbish at Type ...", it is best to just sit down with him again and go through the technique and allow him to answer some of that type of questions in his own time. That restores his confidence, and the next time that they come up in a test he is back on track.