Boxox for minior muscle problems in 4 year old?

My 4 year old son has always had stiff muscles that make it more difficult for him to walk. He's learned to walk on the front of his feet, instead of his heels. We constantly tell him to walk on his heels, and that corrects him until the next time he walks.  He's seen a Phsyical Therapist for a while, and it's helped somewhat. She, as well as the doctor, recommend that he get Botox injections. His case is so minor, it's a hard decision.

My questions:
* Does this type of thing typically get worse with age, or will he be able to correct his walking easier when he gets older?
* Is Botox a reasonable solution for a mild case?

Note: It's not believed to be Cerebral palsy, but I wasn't sure where to post this. He does go a CP therapist.