Hi S.---I don't think I would worry so much about getting too much potassium as I would him not getting different nutrients that he'd get from eating a more balanced diet. Variety is one of the key ingredients to optimal health, and even though most every fruit and veggie has the same vitamins and minerals (in differing ratios), they all have different smaller phytonutrients, such as resveratrol, zeanxanthins, allicins, etc. Those smaller nutrients are basically the colors of the fruits and veggies. So many nutrition experts recommend that you put a rainbow on your plate, as many different colors as possible.
I have some great shopping charts that are broken down into food colors. I'd be happy to share. BTW...bananas actually do NOT constipate. I have a couple of food directories, books that tell what foods are good for what health conditions. My husband broke his hand and collarbone a while ago and of course had that problem because of pain meds. I went to my resources, and 2 of the best foods to help alleviate constipation were bananas and kiwi...it's a wonderful combination.
Good luck...I look forward to 'talking' to you. D.