I have a 13-year-old son named Julian who watches Bob the Builder. This is inexplicable. He is not disabled except for autism, but he also chews a baby's teething ring. He also watches Disney Jr. with 2-year-old Aidan. My god, this is messed up.
If the worst issue you have is that he likes (or even obsesses over) Bob the Builder, count your blessings, and that's before taking into account autism. People of all ages enjoy high quality productions, even if they are marketed to younger audiences. I'm a fan of shonen anime (Japanese cartoons marketed at preteen boys) for example, and I'm on the right side of my half century mark.
People seek comfort in the familiar, and people with autism sometimes take this to an extreme. People with autism also like predictability and routine more than the typical child.
I have a young man close to the same age as your son on my live webcast team who has autism. His passion for predictability and visual balance has been a real boost to the production quality. The toughest part for us fitting him in was finding a place with sufficient repetition where his aptitude could shine.
If your son were watching something less benign, i'd have other advice. Rather than having angst over his choice in entertainment, I'd suggest finding a place he can participate in real life, and a set of ear plugs (13 years of "Bob the Builder" would drive me nuts).
Edit: The adults who create those shows have a passion for them and for kids in general, and they make good money pursuing that passion. It is likely that some of them are on the ASD spectrum as well. It may be worthwhile to open channels with one of the studios and see if he can observe or participate in a production, or your local community TV station.
Edit 2: since posting this answer, I have been diagnosed as autistic (which wasn't even on the books as a diagnosis when I was your sons age). Fancy that.