My little one is 10mths. After she outgrew her baby bath I found bathing her in the sink was easy until she starting trying to get out a month ago, so if your little man is happy playing in the sink you could try that. You can buy little sink mats at the dollar store. Obviously you have to make sure you are right there with them all the time.
Ours wasn't very happy with the transition to big tub but after the third night she was a happy girl again, we just had a few short baths first. Just have lots of toys and it won't be long until he loves it again.