Just to let you know that my DS1 was slower to roll, crawl, pull up and walk!!!

He rolled everywhere from 9 months till 12 months when he started comando crawling and didnt get on hands and knees and crawl till 13 - 14 months and didnt walk till 21 months!!!

(He was treated for a head tilt at 5 months)

Good luck sounds like all going ok for you