Bath tub question

So I have this tub from my sister (not sure what brand) but my LO is >14# and we stopped using the infant sling insert awhile ago and he’s propped up on the slant side. He started thrusting his body and kicking his legs to the point where he can thrust his butt off the little ledge holding his body up. I keep one hand on him like a seat belt but it’s really hard to shampoo and then rinse his hair with one hand. I’m trying to figure out if I need to invest in another infant tub cause I can’t imagine having to keep one hand on him at all time for the next like 4 or so months until he can sit up 100% in the other side of the tub.

Photo for fun! This was a month ago. His legs reach the end of the tub when he launches himself off which sort of braces him and then he has the little numb thing on his back which keeps him afloat. It’s just if he happens to turn his head if the waters too high then he could suck some in :/ he’s a wild one already but it’s good he’s enjoying the bath tub.