My Son Was Diagnosed With Low Muscle Tone At Birth.

My son was diagnosed with low muscle tone at birth. It has improved greatly with physical thearpy. His neurologist is still wanting to do a muscle biopsy at age 2. She said whatever it is, it is mild. He is crawling and pulling up and trying to walk now. He is 16months old. I am just worried that he has something serious. What is your opinion on his situation.
THanks,
Concerned mom