Perhaps he could do a little of all of these since ultimately they will all benefit him and he'll probably have these accounts eventually.
He could open a SIPP now and start funding it and if he continues teaching and joins TPS, he could perhaps roll the SIPP in to the TPS (not sure of their rules). Whatever his career path, he'll still have his SIPP and can keep contributing to it regardless or roll it into a workplace pension further down the line. But this is money he can't touch for 25+ years so he might not want to put all his spare cash into it.
Whilst all the forms of ISA are great savings paths, if he's looking to use his savings sooner rather than later, he might do better to find an account paying good interest, e.g. limited access or regular saver accounts. Then his savings would have a boost he could use towards moving to his own rental. (see above about the £1k threshold for tax on interest)
If he wants to save specifically for buying a property he could open a LISA - he doesn't have to put in the maximum at this time, anything would be a start. But this will be money he can't access except to buy a property and that might not be as helpful right now as having cash in hand.
If he wants to have cash available, since he's very young and his circumstances may change quite a bit in the next few years, he could put some in a cash or s&s ISA - again he can put in as much or as little as he likes. But your point about this being money he's not planning to use for a while is a good one regarding s&s.