It's not the T in itself that does this, it's being subjected to the wrong hormones that does this! For trans women that will be testosterone, and for trans men it will be progesterone (not oestrogen, as you might presume). Having high levels the wrong hormone for our brain type causes a great deal of distress and some of this can be expressed through aggression. So trans women on T might be more aggressive, and trans guys on progesterone or oestrogen can be more aggressive.
When trans men take T, it's supposed to override the ovaries to stop them producing high levels of the wrong hormones that make us feel distressed. But if he's suddenly developed mood swings again, that suggests that he still has ovaries and that they've kicked back into action, producing high levels of oestrogen and/or progesterone again. This can happen even after several years on T. High levels of progesterone are responsible for causing PMS, which can cause mood swings & make one argumentative, anxious, highly emotional, depressed, lethargic, withdrawn, aggressive... sound familiar? PMS is a pretty notorious thing, so why not research the symptoms and see whether you think they match his pattern of behaviour?
So that's my advice to you: ask him to get his doctor to review his levels: not only testosterone, but also oestrogen, progesterone and prolactin. And ask the doctor to review the frequency with which he takes T. Gels give more stable results but his dose might be too low, or if he's taking shots it might be that he needs to increase the frequency to make sure his body doesn't get an opportunity to produce too much of the wrong hormones between doses. Perhaps he might need blockers to knock the ovaries out of action.
Or... perhaps it's nothing to do with hormones at all, and he's just getting stressed about some other aspect of his life?
Quote from: Dani on May 18, 2017, 09:08:12 PM
Testosterone causes aggressive behavior and I counsel many FTM about this danger.
Please don't.
Historically, it's been mostly MtF patients that have attended GICs so Gender Therapists have consistently heard report after report from trans women who say they were much more aggressive on nasty ol' testosterone. So the therapists see this as a worrying trend & they worry that giving T to FtM patients will make them aggressive too.
So imagine everyone's surprise when they find that it's actually incredibly rare for an FtM patient to become more aggressive on T; in fact, most of us mellow out! And the ones who do become more aggressive had aggressive tendencies to start with. Trans guys around the world are struggling against this misapprehension: I know you probably mean well but please don't add to the problem. After all, if most trans patients were FtM then trans women would be getting warned about becoming more aggressive on oestrogen.

TL;DR: People who are aggressive aren't aggressive because of testosterone: they're aggressive because they're aggressive.