I would also put LO on tummy time as well as it will strengthen his hands and shoulder that are required for crawling. You can also get him to reach for toys in a prone position. They always start by creeping forwards first before going into 4 point crawling.
With encouraging bilateral use of hands, I will try bigger objects to hold so they have to use 2 hands. Think the milk bottle idea suggested is a good one cause food is always a good motivation.
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