I assume that slapping him back is an attempt to show him what it's like to be on the receiving end , but he's too young - and it really does set a bad example to children.
I know it's frustrating. We went through hitting, kicking, throwing toys and other stuff around and it lasted for AGES !!! - Sorry, but for us it did.
Personally, what worked for us was spending some time playing (be COMPLETELY focussed on your DS) and praise the 'lovely', 'gentle', 'anything that is not hitting' behaviour - be REALLY over the top happy when you praise him. Then tell him 5 mins and you will need to wash-up (or something). Then give him 1 min warning. Then remove yourself for a short time to do said chore. Likely he will then throw a wobbler (but he may not, and you must praise him if he doesn't). If he does, ignore tantrum/screaming, but if he hits or kicks (or whatever is unacceptable to you), than kneel down, look him in the eye (gently hold his arms by his sides so you don't get whacked), and firmly say 'No, we do not hit because it hurts.' Don't shout at him. If he throws the wobbler when you give the warnings or when you are playing then obviously step in with the 'No, etc' at the time. Then straight away we would change tone of voice to a cheery one and try to distract him with a toy or something. This kind of felt wrong at the time, like I was rewarding the bad behaviour, but it did work because it diffused the tension rather than entering into a battle of wills.
He will push and push and test and test this routine, but I really think it worked for us where everything else we tried failed. I think rewards and stickers can sometimes complicate things with very little children and he is still very little. They really do learn by imitation at this stage in their development.
Hope all that doesn't sound too preachy. I don't often reply to the behaviour posts in this kind of detail, but it all sounds sooooo familiar .
p.s. I did lose my temper once when DS slapped me relly hard in the face, and I smacked his leg (bare as I was trying to get him dressed st the time). This was months ago and he STILL occasionally hits me and then shouts at me to 'hit his leg'. It really does make a huge impression on them and takes time for them to unlearn it, IYKWIM.