M.S.
It may be something that a pediatrician needs to look at, but from experience it may be ok... My daughter is a big girl and was slow to develop as well... We put her in daycare at about nine months and she was not even sitting up really well... She would lose her balance and fall easily... She did not crawl and never pulled up on anything... She developed at her own pace according to her doc... Now she is almost two... She didnt start walking until about a month ago... Every child develops at their own stages... My boys walked before they were one years old... My daughter wasnt interested in that... She was perfectly content with what was in front of her... I hope this helped a little... Just keep working with him... You may try the stationary Gyms... They are like walkers that dont move... We moved my daughters low enough that she had to put her weight on her legs... It did help for her strength... I hope this helped a little bit... And good luck :)