Hi,
You can take Echinacea in tablet form, but the reason I suggested the tincture is that it is better absorbed by the body.
Acidophilis is a mixture of the good bacteria that naturally colonize the gut, so will do no harm to the antibiotics. Should be ok if he takes the antibiotic at different times to the Acidophilis. Probably only need a couple of these daily or it is probably better to wait until he has finished the course and then take them to help re-colonize what the antibiotics have killed off.
I think there are sites that list American based homeopaths. Not sure whether there is an American Society, but there is a website that lists ND's who practice homeopathy. Dana Ullman's site might be a good starting point, or
http://www.homeopathyhome.com, a site that list homeopathic resources. I think you can also search for practitioner in Britain and the US.
Anyway, hope this helps and good luck?
Sarah