“but I’m not sure that he really knows me well enough.” This is reason enough NOT to have him send in a positive but frankly, extraneous and unhelpful rec letter. It’ll only water down the other rec letters from enthused teachers who know you well. Being an online course, the instructor didn’t see you interact w/other classmates. Think about what he can say (ignore his pedigree – his employer or what schools conferred his diplomas – they are irrelevant) – it’s not much, IMHO.
Omit it.