I had to go a quick conversion there as we use litre tubs.
36qt looks to be 40litres; many large females here are kept in a 33l tub at adulthood. I think this would depend if your tub had more floorspace than height. If so I would think he should be okay in it for life. You could upgrade to a 43qt which would be a 50litre here. I know some people keep their adults in this size too.
Sorry for the shoddy conversions! I only have hatchlings and juvi's so was doing a quick work out for you. I may be wrong