Hello. For me I wouldn't worry too much esp if he can do other skills/milestones for his age.
Kasi sa baby ko hindi rin natuto magcrawl using hands and knees at hindi rin natuto umupo from crawling position. Pero natuto na siya tumayo at 8 months, and she prefers standing up and walking around the bed or playpen 😅 Kapag gusto niya umupo, she would stand up first then uupo 😅 at 11 months marunong na siya mag walk magisa.