Well it's clearly a legal issue, since to do the M->SSEA upgrade his needs a current licence, so he needs to revalidate (presumably by test) on 3-axis microlights before he can do his SSEA GST. Daft I know, but it's the rule and other people have been caught out on it. Best bet is that he revalidates at a convenient juncture in something like a Eurostar that'll be pretty similar to what I'd guess you're likely to be training him on. You might want to check that his 3-axis differences training was formally signed off as well, otherwise it'll probably vanish into some awful paperwork mess.
Engines
2-stroke Rotax engines have a sufficiently high air-mass throughput that they don't have any real tendency to ice. Where a carb heater is fitted on those it's normally electric - start engine, turn on, forget about it.
4-stroke Rotax engines usually have heated carbs, with a sort of shroud of hot coolant.
4-stroke Jabiru engines may occasionally have a "conventional" carb heater, or more likely an electric one, or sometimes none at-all.
Conventional wet-bowl carbs is most common, fuel injection very rare. A (very) few 912s have mixture control but that is only likely to be for tweaking cruise mixture and can't kill the engine. Rotax and Jabiru engines are invariably turned off on the ignition switch(es), not on the mixture, which can't. Pretty much all Rotax or Jabiru engines need use of a choke control to start, but the exact use of that can vary significantly between models.
Flexwings
In general, flexwing pilots seem to transfer to 3-axis quicker and easier than the reverse. Control reversal will probably not be an issue, but do be aware that he's used to flying on a foot-throttle, may well never have used flaps, and is used to what (to you) will be staggeringly short take-off times and distances so is probably not used to having to actively control the take-off down any kind of significant runway distance. He's probably never flown a yoke, but that's as trivial an issue as it is between yokes and sticks within SEP. Most flexwings and simpler 3-axis aeroplanes also generally only have a single trim condition (or at best a near-useless trimmer) and you maintain level flight by setting power and accepting the trim speed. So, the odds are that your man will have little instinct to trim.
Flexwings and older 3-axis microlights also have very low instrument panels, so pilots tend to have less of an instinct to pick a horizon reference than group A pilots will be trained to.
Checks
He's almost certainly learned to do all checks from mnemonics - which is good, but they'll be different mnemonics to the ones you're used to. Pre-take-off checks normally fully incorporate engine run-ups, he's probably not used to them being distinct and separate as they usually are in group A.
You may need a few more training hours than I will with my 400ish hrs 3-axis student!
Hope that helps a bit, I look forward to hearing how it goes.
G