Yes, you should definitely start to fertilize to encourage the new growth. And you can repot if needed if you don't need to do major root disturbance.

Even if you think it needs repotting, I would not do it now, next spring will be soon enough. Right now you want to keep it growing. Most of the flowers will fall off without forming fruit which is a good thing right now. Yes start fertilizing now and keep it up monthly. About a tablespoon of citrus labelled plant food every month. Citrus grow year around, and in a container they need to be fed year around. Al

I'm not sure what you mean about root disturbance. It's in a small planter right now, the one that came from the nursery. Maybe a 3 gallon? I think it's smaller than the 5 gallon pot that my patio tomato is in...

If you bought your citrus this year in a 3 gallon container, which is a common size for retailing citrus, the size will be OK at least till next year. I would not recommend repotting your struggling tree now. Al

OK, I'll get to fertilizing. Bummer it won't produce fruit this year, but it sure is pretty right now with all those little blossoms! Gonna be smelling great around here if they all open, so at least I've got that :)

Even if it manages to form fruit this year, I would cut them off early so it doesnt spend any energy on them. They will probably be terrible from a stressed plant anyway.

Oh it will be so sad to pluck those little baby lemons! But ok. The tree has a wayward branch that's about a cm above the graft line. Should I prune that off?

I like my citrus close to the ground, just not touching. I would shorten it to keep the tree shape. Al