If he specifically wants to build up his dips that you could try:
1) Place a box, step, or other sturdy raise surface in the pull-up/dip station high enough so that he can stand on it with his arms fully extended (like he's at the top of a dip). Then have him perform the dipping motion, but use his legs to assist him in completing his reps. Also, have him really focus on form here and make sure that he's trying to perform his dips exactly like he would without the assistance of his legs.
These could also be combined with dip supports, where he would just pick up his feet (if using a box,step, etc...) or jump up into a dip and try to hold the top support position for time. I'd suggest using about 50% of his max hold time for as many sets at it takes to reach 60 total seconds of hold time. This will help to strengthen up his connective tissues in his elbows/wrists and get him used to supporting his weight. Hold off on these though until he can hold the support position for at least 15 seconds.
At first he may need to use his legs a fair bit for assistance, but with time he'll need to use them less and less. Once he needs minimum assistance move on to
2) Negative BW dips. Probably familiar with these, but if not have him jump to the top of a dip and then try to slowly lower himself to the bottom of a dip. At this point he'll put his feet down to catch himself and then repeat for the desired number of reps. He should eventually strive to be able to lower himself under complete control for a full range of motion in about 5 seconds.
Once he can do say 3 sets of 5 negative dips with a 5 second negative, have him move on to
3) Bodyweight dips. He may only be able to do 2-3 at first, but with continued practice his rep numbers will improve.
Once bodyweight dips become fairly easy he can
1) add weight
2) move on to more difficult versions of bodyweight dips