D.B.
There are inflatable tubs, mostly for travel, out there. We have one and on a cruise with no tube bathed dear baby in it. Packs up so nice and out with little blow up time. It could be more like a pool and put it on one of those
Neat Sheets that are water repellent and he could bath anywhere at first moving closer and closer to the bathroom and the tub. Has he tried a bath with Daddy just play time with splashing and water approved toys. Reading a plastic tub book or blowing bubbles? Good Luck!