They are all so different.

Is he doing other things? rolling, reaching for toys, babbling etc??

DS1 didnt crawl till 13 months, he didnt walk til 21 months. He now at 28 months is always running, climbing etc.
DS2 is not interested in crawling or anything he can roll but when laid on floor he cracks it very quickly and wants to sit up. If sitting next to us he can pull up on our clothing to a stand.

I hope all is ok.