I've got a tomato plant (actually this one is my six year old son's) that looks like it's starting to show some problems. Overall the plant looks healthy. It's growing very well (better than mine) and has started to set fruit. This morning, I noticed that a lot of the leaves (mostly lower / older leaves) have small brown spots. The spots appear most visible on the underside of the leaves but some show through to the upper side as well.

I removed all of the branches I could see with spotted leaves. Afterward I sprayed with a garlic/baking soda spray that I already had on hand. I'm not really sure what the problem is (guessing fungal but I'm still very new to growing), so any help identifying and determining the best treatment would be wonderful.

(In case it helps with any recommendations, skipping chemical treatments in favor of home made solutions would probably be preferred, but I'm not trying to be strictly organic.)

Thanks!