Hi Pam, sounds like he's using one of those cheap plastic mitre boxes which will drive anyone crazy with all the wobble, etc. The best thing would be a good power mitre saw (i.e., Delta, DeWalt, Makita, etc.), you'll spend $180+ but it's a real good investment. Another good investment would be to buy a new tape measure so that he can get good accurate measurements and it's only going to cost a few extra buck. Hope this helps, and good luck!
WickRock Chalk Jayhawks, KU!
P.S. Just tell him Father's day came a little early this year. ;-)