My 5 year old chews on his shirts. Which toughness level should I get him?

The soft/standard level is usually the best option for those who like to chew on shirts/fabric. 

Unless they chew holes in them regularly, in which case the XT (possibly XXT with very avid chewing) usually holds up to that better. 

As far as which shape to go with - we like to recommend the Chew Tags, Arrowhead, and/or Krypto-Bite Tube as substitutes for shirt/fabric chewing.