I think the resource he may be guarding is Meeka, and maybe you too now that he has been with you longer and may feel he is in charge and needs to protect you. I would start at home with some basic obedience, sit for cookies, down for a treat, etc, just a few minutes a day to reinforce that you are in charge, gently. If it is possible go to the park with him alone on a day or time that isn't too busy, or try the small dog side (if there is one) and see how he does. If he plays, or is neutral, give him lots of "good boy" and pets, and try your best to be relaxed as well. They really key off your emotions.."gosh, Mom's nervous and frightened, I better protect her!"…so be all happy and cool, lets go play, etc...but stay close to him and be ready to grab him and leave if he acts aggressive.
Tanza knows more than I do, but if he and Meeka eat well together now, I wouldn't change that , or surely not make it into a big deal. If it is getting hotter, he may just not be as hungry. And a full thyroid panel is in order, a low thyroid can change the personality and make for a very grumpy dog, easily corrected with a pill a day.
Good luck, we have worked through dog park issues over the years and for us, leaving immediately when really bad behavior happens is a start, and lots of praise for good behavior. We are lucky, my dogs have their 'regular' dog pals and so are much more relaxed that when they are there alone.
Good luck!
Anne in Tampa