Use Beaphar bone feeder & home food plus with food. Also note at the time of growth many pup feel like they are thin but actually it's growing so it look like thin.

1)Home food plus 1/2 spoon (Morning and night) 2) Beaphar - HD tablet 1/2 tablet night 3) Bone builder - 1/2 spoon Morning All with food

For how much you have purchased your lab

And always give him skycal calcium..... It's must for every dog

You can give skycal and proviboost 5 ml morning and evening. Do not give him milk. Feed curd rice carrots and peas it will be helpful.

After reading all comments, it seems like the legs are thin due to genetic problems. His parents might have weak legs. As per my knowledge, you cannot make them thicker but can strengthen them with calcium supplements and daily workouts.

Buy CF pets hip joint support suppliments, 599 on Flipkart give 1.5 per day half for every meal and continue till it's finished , if has good results then consult vet to continue or not