My nine year old autistic child sounds very similar. He has rigid interests but he is dyspraxic which limits his ability to access even things like lego.
DS had a lava lamp for his birthday and adored it...so I definitely second that!
He has asked for a rad robot this year. It's only £15 , it looks like total tat....it's a small ball that can record your voice and play it back with funny voices...anyway totally rubbish but he thinks he'll like it!
He will have a nerf gun which he loves.
He also loves minecraft, but has been into it for years, so like your son has tonnes of the merchandise already, but I found the Minecraft mobestiary book for 3.60 at the works so he's getting that too.
He quite likes board games and played labyrinth at a friend's and enjoyed it so that's also on the list.
You don't mention if he likes video games, but my ds adores his Minecraft Xbox controller. I'm also considering a gaming chair as I think he will love that.
Hope that helps - I have four children and he is by far the most difficult