How long has he been gone for? Mine started doing this at about 11 months. As I type he is missing now and has been for 24 hours.
I tend to go out once the streets have quietened down a bit and call him. Sometimes I find him, sometimes I don't and he rocks up later. If neighbours have gone away, try calling outside their garages (inquisitive youngsters WILL get themselves shut in), if you have open spaces near you try walking through there and calling. Unfortunately, I always end up checking the main road too - just to satisfy myself he is not there.
The longest he went for was 4 days.