I would recommend he goes for general service chef. This would allow him to spend some time in the RN having a good time, seeing the world on surface ships. Once he's got bored, having been to all the good runs ashore, if he is still interested in submarines he could then volunteer to go to boats. It is likely that they would then snap his hand off.
The other option is to join as a chef SM and go straight to boats. Once in boats it is considerably more difficult to move across to GS so it would be likely that he would be permanently a submariner.
Of course, I'm no expert in chef branch progression but I believe there are some branches which once you reach a certain rate you can no longer serve in submarines so by default come across to GS. I imagine chef may be one of these but this is most likely at CPO level. I know an MA and Logs(SC) who both left submarines to have a sea draft as a CPO.
Short answer: it is easier to go GS to SM than it is to go SM to GS
