Q. Why is my 6-year-old son is loosing weight and have knee clicking?

Hi doctor,

My son is 6-years-old. His height is 1.5 feet, and his weight is 44 lbs. He is becoming thin for the last two years despite eating well. Over the previous six months, he has had clicking sounds from his knees during any knee movement. I have seen such knee clicks in people above 60 years of age. He keeps many things in his mouth and puts his hand most of the time in his mouth. He was sick two years ago and started becoming thin after that. We checked his CBC, and everything looks normal. His TSH is slightly high. His vitamin D level was slightly low, and we are giving supplements. Now, vitamin D came back to normal two months ago. He drinks one cup of milk on average every day. We still hear the clicking sound, and I sense his knee strength is low as we see he cannot balance well when he jumps. He is not telling of any pain and also no swelling.

My questions are as follow;

1) Could there be some bacterial or parasite infection causing his metabolism high?

2) Is knee clicking sound normal in some kids?

3) Could there be some infection impacting his joints and metabolism with no other symptoms like a fever?

4) Is there any component in CBC that indicates long-term hidden bacterial, viral, or parasite infection?

5) Does he require any additional tests?

Kindly help.