In your shoes, I'd proceed on the diagnostic pathway 
Maybe they aren't mood swings he has, maybe they are his reaction to a trigger or different triggers which so far you're not seeing or understanding is a trigger for him.
All behaviour is communication, so it's possible that at his age hormones are playing a part as well as anything else.
Also the transition to secondary can be hell for a neurodiverse kid, at primary they had one teacher all day and were mostly in one room or other activities in different rooms were close by and they knew what needed doing and where, so think mostly orderly repetitive lifestyle with few surprises or major changes.
Secondary is a whole new ball game, comparatively massive campus,
books and kit to be remembered, found and transported to a different room for every lesson with a different teacher each time and sometimes different kids too, unsupervised and unstructured breaks, social norms in their peer group they can't understand, you get the picture. Imagine how hard that is for a neurodiverse person to navigate and that's before they've got their anxiety lowered enough to sit on a chair and try to pay attention.
Very often, undiagnosed neurodiverse kids present with extreme behaviour after starting secondary or at a point where they can't cope because the pressure on them is too much.
he can be absolutely furiously angry, then a couple of hours later it's all forgotten and he's charming and affectionate. It's incredibly hard to deal with
The furiously angry behaviour could be pent-up frustration of many sensory and physical things then when it gets to a certain level, like a frequently-shaken coke-bottle, he explodes and once he's recovered his equilibrium through that release, he's himself again.
If it is that, you won't know how the many triggers he's suppressed for hours previously have wound him up to that point.
Or there could be PDA in the mix. again, try the strategies suggested for that to see if they work, at this stage it's not a question of assigning a behaviour to any trait, more like trying to understand what triggers there are and putting interventions into place for him so he doesn't feel there's no alternative to react in an explosive way.
Good Luck, it's a long road to walk and find the right support 