Firstly, I agree - as usual - with Cherie. Assuming 'properly bitted up' means properly trained & ridden.
I would not like to assume, from that little info you provided, what the problem was though.
It could be to do just with having a couple of months off from you, whether that's to do with lack of exercise/overfed so too much energy, or that your friends haven't worked with him in the same way as you, so they've inadvertently taught him 'bad manners' or such.
He's a youngster & perhaps still growing physically and just because there are no 'loose bolts' or actual saddle sores(that only tends to happen with chronic/acute saddle issues) doesn't necessarily mean the saddle's comfortable & properly fitting, even assuming it was a few months ago. He may have had a 'growth spurt', but even mature horses change shape over time & seasons, so *proper* saddle fit checks are important to do regularly. He may have other pain/discomfort issues; teeth, hooves, back, neck.... etc.
He may not have had a very good education at the start, or may have been 'broken' with rough treatment originally and when you were riding him regularly, he was kind of 'shut down' and just acting like a robot. After a couple of months off, he's come to life & showing his true personality/confusion/fear. I think that's why it used to be so common in the days when 'breaking' horses was the norm, for people to believe you had to ride a horse regularly or they'd 'go feral' & have to be 'rebroken'.
It could be something about you. Perhaps you've got a physical problem that causes you to be imbalanced or such, making him uncomfortable & reactive. Perhaps you have an emotional problem - something's happened lately, or it's a long term problem that he got used to in the past...
So, not assuming any of the above, just giving a few possibilities, that I think are important to consider, before just treating it as a straight training issue.