What Is The Treatment For Dark Spots On Trunk With Lyme Disease?

My 12 y/o son developed a spot of what looked like poison ivy on his trunk yesterday. He also has some spots on his face. This morning the spot on his trunk is darker and has a very faint red spot around it. He had Lyme disease several years ago which we initially mistook for poison ivy for about a week prior to the development of the bullseye. We live in CT, snd he has spent much time outdoors this summer in CT and NH. Thinking we should have this checked, but wonder if there is any harm in waiting until Monday (it is now Saturday fternoon)