"Active" 20 Month Old

If every time he touched a lamp in your house and that lamp shocked him then you can bet he would learn very quickly not to touch that lamp. That's because a shock feels very unpleasant and it happens at exactly the time he touches the lamp so he knows that the lamp is responsible.

When he bites, he needs a stinging pop to the cheek. He needs to feel a very bad feeling right next to his mouth when he is biting and it can't be afterward, it needs to be in the moment. Stay nearby and and wait until you see him start to go for it then swoop in with the pop.

If a lamp had shocked him you wouldn't be apologizing and taking the blame, you would say "oh you got shocked by that lamp! that's too bad, you need to stay away from it." so don't take responsibility for popping him and say that you're sorry you had to and all that... Instead act like you didn't do it and that its a natural consequence of biting, such as, "oh no, you bit her and you got a pop! you'd better not bite any more".

What discipline methods are you using? Timeouts...? Consequences vs rewards?

I guess I'm a bit old school due to I have a 16 year old and a 20 month old. When our daughter hit, I hit her back, when she bit me I bit her back. There has been no more hitting or biting. Yes, I believe in tapping her hand if need be or placing her in the time out chair. It appears your son may need additional help with being social with other kids. Yes, you should definitely work on that before placing him in a daycare. Have you tried putting him in gym activities that involve mommy and me classes or mom and tot? You can definitely help stop your son's biting and hitting; it will just take a little time and making sure that you stick to your guns. Loving yet firm.
No worries you'll do fine and you'll remember when it was just a phase.

What do you do when he bites?
It needs to be consistent and unpleasant for him every time. He also needs to say "sorry" to the other child.

From what you write, he seems ready for some social interaction, and lots of physical activity. My son acts totally differently for his childcare provider than he does for me -- he knows what side his bread's buttered on:)

Perhaps he acts out when tired, or when he cannot get his way and is angry. He might act out with girls because of their differences in play and interaction -- they aren't large but they are there:) Maybe he can show that anger by stomping or jumping to release tension as he better learns to talk about his feelings?

Be firm, and read up on different "modifiers" -- each kid's different and he might be like my boy, where force (pops) make him more defiant or willful, but "quiet time" in his room sat in his bed gives him space to calm down, get out of the "heat of the moment" and better if not totally understand.

Good luck redirecting that "physical" little boy -- wish me some too:)