I agree, you can buy a slip-on metal reflector on Amazon.com that matches the length of your UVB tube, or you can try to make one with that heavy'duty tin foil...OR, if you have some tin-snips or shears, just go to Lowes or Home Depot and buy a cheap sheet of the flexible aluminum/tin and cut it/shape it to go around the back of (behind) the tube and attach it. It's exactly the same thing that the commercially sold slip-on reflectors are made of.
It's very important though, because right now you're losing 50% of the
UVb light off of the back glass, since it's not centered in the middle of the top of the tank, and then in order for him to get any
UVB light he has to be right directly underneath the tube right now. Try to move the tube fixture away from the back of the tank and more towards the center, and then do either buy or make a metal reflector that goes behind the tube and then wraps around the back of it, about 1/2 inch away from it. This will reflect the
UVb light all throughout his tank, rather than losing it off of the back glass and him having to be right underneath it to get any.