He isn't an ISTJ or ESTJ - ESTJ is one of my closer coworkers and I have an ISTJ wife. They are even more rigid and strict than ENTJs. They wouldn't let that happen in the first place - or they would behead the kid as soon as they found out about it (they tend to have a zero tolerance policy).
I almost wanted to say ISTP tainted with a "I'm a rural man with a gun" type attitude. It could explain his interest in IT and guns which sound a bit more enthusiastic than the average person. ISTPs are generally more relaxed and his daughter may have taken advantage of the situation. And the gun thing could be his way of non-verbally communicating his anger.