I think my baby's muscular development is behind?

I have two kids of my own one who is 9 months and one who is 7 years. I also have two step kids. They all did these things at different ages. I would pesonally wait a little longer and see what happens. My daughter is 9 months and still only drinks out of her sippy if I hold it. Some kids don't like to feeling of weight on their legs and feet and start doing those things later. if anything I think maybe it's a sensory issue rather than muscular. If he is scooting then he probably has good muscular development. Good luck and remember if you are really worried there is never any harm in talking to the doctor.