That's about the same as my Daisy. The growth rates vary a lot. Try daily soaks, humidity, humid hide boxes and adding some Mazuri to the diet 2 or 3 times a week. Keep your tort warm (75-80, round the clock) with all the humidity and watch him start having a growth spurt.
Daisy's growth has taken off since I started this routine. She's 6.5" now at three years. A lot of that growth is just from the last few months with this new routine.
A lot of young tortoises and very dehydrated and fed a diet too low in protein according to people much more knowledgeable and experienced than me.