That's not good Derek, after your first ride (when it's burning off any excess oil used during the assembly) a properly built engine with correct piston-to-cylinder clearances shouldn't be burning any oil, period.
Pull your plugs and examine them, if they look "oily black" as opposed to "rich mixture black", do a compression check, with a fresh 836cc engine you'd expect upwards of 180 psi per cylinder, with a 5% max variation.
If it's less than that, pour a little oil in each cylinder via the spark plug hole and check again. If the compression figures come up, it's your piston-to-cylinder clearance that's at issue, or if it's just one or two cylinders you might have some busted rings due to poor assembly technique. If there's no difference at all, it's a valve/valve guide sealing problem.
My advice, for what it's worth, is to take it to somebody other than your man who built it initially, I suspect that he's no expert..............