Phase 1
Realize that this is a very common situation; it is easier to deal with than the other extreme, if he is doing an effective job of cleaning himself up after the bathroom you are ahead of the game.
Phase 2 (I will play devils advocate for this one)
Lets see, he “has” to do all these things that you talk about, well the human species survived until recent history without any of these things and even in current time there is a large part of the world population who “survives” without any of these “social niceties”. English culture is particularly “phobic” about “smells” and cleanliness, as we are consumed by endless other superficial social areas like fashion, appearance, the newest trendiest item and so forth while we only give a small percentage of that level of thought and efforts to the trivializes of injustice, discrimination, famine, war or just helping others in our own community who are in need. And look at all the benefits of hygiene, you can be in crowed places and people will want to come close to you, talk and socialize with you (typically about social trivialities), wow !!! every aspies’ “dream”. Oh yea a 10 minute shower is a bout 2x as long as it takes to get clean if your are “efficient”, and yes the “repeat” part of “lather, rinse, repeat” shampoo instruction is just a way to get you to use more product unless your hair is exceptionally dirty.
Phase 3 (done with the aspie sarcasm, for the most part)
The first thing to do is to stop thinking like an NT and let go of the overwhelming drive to dive to socialize as the basis for the world. I know this is hard since it is hard wired in and./or ingrained from birth and as the NASA extended space travel studies have shown a rapid determination in mental health occurs with out it for NTs. Then do some analysis with your child about the “true” impact of hygiene on his life. Social superficialities are rarely part of the equation, even his interest in “girlfriends” at his age may not have much practical impact, especially since it is common for them to be a “girl friend” long before they become a romantic girlfriend
As always take sensory sensitivities into account, and adjust for them as possible.
You need to find a way for him to “buy into” the process of “socially standard hygiene” (to the greatest extent that he is willing to). After that depending on EF issues you he may need some supports (a step by step check off chart in the bathroom is one of the best ways).
My DS9 Aspergers, has always had a difficult time with tooth brushing, both the texture and taste of toothpaste is an issue. Our dentist decides that brushing without toothpaste for a reasonable amount of time was more beneficial than very short times with toothpaste. Of course we had over time gone over all benefits of tooth brushing and it helped a little but could not overcome the sensory issue. One day last summer there was a show on the TV about dental care, which was very direct and informative, DS watched it but I did not think much of it. The next day I had to stop him from brushing (with toothpaste) after about 10 minutes. Obviously there was something in the show that was “logical and applicable” to him, you never know what will do the trick.
As to a book “Freaks, Geeks & Aspergers Syndrome by Luke Jackson might be something you want to look at. Really the better informed he becomes about what it is at the core to be Aspergers (or an aspie) the better able he will be to make thoughtful and informed choices about how he chooses to adapt to Neurotypical society. For this peer support groups are good and as always Tony Attwood’s “The Complete Guide to Aspergers C2007 is a must read when he is mature enough to put it in “perspective”.
bookwormde
DS 8 is the opposite. He's one clean kid. 
