Don't be so hard on yourself
I definitely felt the same way. I kept thinking that if I had eaten better or breastfed that he wouldn't suffer, etc. This is new territory for us, we aren't doctors, so we do the best we can, and I think we are doing great!
I think BLW is great. I tried it with my son, but he had such a gag reflex that it wasn't until 2 months ago that he started eating more solid type food, like bread. Now he's doing great, but if you can do it I'd recommend it. Especially where you are just going to give him straight fruits and veggies, I'd say why not. He's still young enough to learn and it's about learning at this point anyway.
What kind of formula is he on? I'm wondering if it was just a delayed reaction to the formula? You said he just started the formula in the last couple of days?