New owner to a reactive 6 year old dog, advice needed for walks

I recently adopted a 6 year old dog, who is fully house and potty trained and is a sweetheart. He does very well with new people, but is extremely reactive to other dogs, even just seeing them in the window. We live in an apartment complex, and it makes going for walks extremely stressful and difficult.

I am in contact with a few trainers and evaluating options, but in the interim, does anyone have any advice? I’ve honestly been trying to just walk him very early or late when we are less likely to see another dog, but we have still run into a few and he goes absolutely crazy.