I am a mother of a 4yr old boy and 2yr old girl and we did the same with both because hand washing time was a huge mess! We practiced a little song to help teach us what to do away from the sink just in everyday play and then applied it one day while washing hands and it worked like a charm!
Rub Rub Rub your hands as slowly as you can
Rub Rub Rub your hands as quickly as you can
Clap Clap Clap your hands as ...slowly... quickly
Roll roll Roll your hands as ...slowly... quickly...
Shake Shake Shake your hands as... slowly... quickly
Wiggle Wiggle Wiggle your fingers as slowly ...quickly
Once my two had the little song and moves down I applied it while they where washing their hands....
Fill sink with water...
Let him get one pump of soap.... (Kandu my kids loved in the beginning now I let them pick whatever hand soap they like at the store)
then sing your little song and do it with him in the beginning rubbing your hands together change the song up if you need to to get him to wash the tops and bottoms of his hands... I would just sing the rub and wiggle parts and apply them to the washing but do what works best. I would also have my kids clean up any water they got out of the sink.
After a while they would be no mess and they where so proud of themselves. Now at 2 and 4 they do this completly unsupervised. Son fills the sink and washes first then daugher goes next...they'll clean up their mess and come show me tops and bottoms with grins from ear to ear! Every now and then I hear them in there singing our little tune!
Good luck