I'm going to pick on my friend till he tackles me
What is your reason for wanting to pick on your friend until he tackles you?
Because he told me he could beat me up.
It's all fun and games until someone gets hurt. You shouldn't provoke your friend because he obviously doesn't want to tackle you. You should respect your friend's boundaries and not annoy him past breaking point because it's rude and disrespectful.
What if I wasn't joking, I can take him down.
Trying to pick a fight with your friend just sounds like a bad idea. If you want to see who could take who down, you may as well just agree to it and go ahead and fight. At least this way nobody is caught off guard.