At his age he should have a kidney test. If his kidney function is down with age, his body won't retain potassium and that affects his muscles. This happened to my Carney. At 4 months, he collapsed and couldn't stand. Turned out, he'd been born with malfunctioning kidneys and it showed up by 4 months. He gets a kidney powder and potassium powder every day... I mix them with 1.5 cc. water and syringe it into him. He's been healthy ever since. He's about 7 yrs old now!!! So if the weak muscles are potassium related, it's definitely treatable!