OP, it's worth taking a deep breath and looking up the legal definition of what these offences mean, because it's a bit different to what most of us think when we hear assault or battery.
There are a range of assault offences, going from common assault, right up to causing GBH with intent. The differences between them are the seriousness of the injury caused, and the seriousness of the injury intended. At the lowest, common assault is committed if you make someone afraid you will be violent - you don't even need to touch them to have committed an offence. Yelling at someone in a pub could be enough, if you made them think a punch might follow. Battery is committed when you touch someone, almost no matter how little force you use. So going from shouting at them to pushing them, for example. If he'd caused a meaningful injury then he wouldn't have been charged with battery, because a more serious offence would apply. Both of these are 100% consistent with what he's told you - that he has a history of fighting, and brushes with the law in consequence.
Theft is similar. It could be everything from impulse shoplifting to a major bank heist.
False imprisonment is more serious, and I entirely agree you need a full explanation from him around that, particularly if he was actually convicted.
Do you have information on the sentences he received? If so, these will give you a really good idea of how serious the courts considered these offences. If it's a string of conditional discharges or small fines then the story is consistent with what he told you. He was a wrong'un in his early 20s, and then grew up. If there are multiple prison sentences/suspended sentences then it was clearly more serious. The sentencing council publishes guidance on the seriousness of offences and what sentences should follow and, if you look at that guidance, you will be able to sense check how serious they actually were. Google will find it.
I guess all I'm saying is that, whilst I appreciate what a shock this is, it may not be as out of line with what he told you as it came across when you first read the information.