I would say on average he is getting about 12-12.5 hrs sleep in a 24hr period, does this seem okay?
For some LOs this is abosolutely fine. Both my DDs got/get about this at this age and it's definitely not unheard of. However, for others it is too little. I think the biggest indicator is his mood. Does he seem well-rested or does he seem tired? If he's seems generally happy on this amount of sleep then it's most likely fine.
It's tricky to tell though when there are other things going on (like teething). When do the tantrums usually happen? At any time or more when you'd expect him to be tired (i.e. close to nap/BT)? Teething/development/OS can all be other things that can bring about tantrums so it's not necessarily tiredness, although tiredness is of course a big cause of tantrums. Sorry, that's not too helpful really - there are so many factors!
he goes straight to sleep but wakes up hysterical so could be tired still or in tooth pain?
My DDs always woke/wake crying from naps, even when they've had enough sleep and aren't in pain. Some LOs are like this. It could be tooth pain. Or just the transistion from sleep to awake is tougher for some LOs. If my DD2 wakes early from a nap (usually due to teeth, at 30-45 mins) and is still tired then she will go back to sleep if I sit next to her, sometimes with a hand on her bottom. If she's not tired or the pain too bad, then she won't resettle (she won't lie down, she sits or stands in her cot). But usually she will if she's tired enough.
In the past couple of days I've tried another nap at 4ish for him to catch up but he blankly refuses so I guess he can't be that tired!
When my DDs need to catch up a bit I always find EBT works better than an extra nap. If they have (had) an extra nap (usually only if we're out in the car and they nod off) then this just delays BT and so they don't get any extra sleep in the end (or they end up with less sleep because a 5-10 min CN can push BT back by 30 mins). With EBT though, if they need it, they get that extra sleep at the start of the night and still have their usual WU time.
HTH