i think i would give him until the suggested 6months, in that time hopefully he'll stop feeding in the night, and then move him in with harrison and by then hopefully they'll both settle in well together
we moved lewis in with lily-ann at 5months ish and it took them a while but now they if one cries, the other sleeps through it. we also still give lewis a bottle at midnight(he doesn't have one in the day so it's getting him his milk needs) and that sees him right round through until morning