Excellent photos. The back lesions are epidermal collarettes which usually indicate pyoderma - bacterial skin infection often caused by Staphylococcus. The roughly circular collarettes begin as pustules and then spread outward. The center quiets down and often hyperpigments but the rim can remain actively inflamed, infected, and crusted. Collarettes can also be seen with fungal infections and autoimmune skin diseases. Treatment at this time should consist of an antibiotic in the cephalosporin class such as cephalexin for 3-4 weeks and 1 week past clinical signs. Bathing every other day in a 2-4% chlorhexidine- or 2.5-3% benzoyl peroxide-containing shampoo should hasten resolution of the infection. The antibiotic must be prescribed by an attending vet. You can find the shampoos over the counter where dog supplies are sold.
The areas at top of his legs represent skin fold pyoderma which can be treated similarly but I'll post my entire synopsis of skin fold pyoderma for you. You'll see some redundancy with what I've already posted.
1) A weight reduction program should be initiated if Rudi is obese.
2) Cleansing wipes (i.e., alcohol-free acne pads, baby wipes, chlorhexidine-containing pledgets, other antimicrobial wipes) used every 12-72 hours work very well.
3) Alternatively, routine topical therapy can be used to control the skin problem. The affected area should be cleaned every 1-3 days as needed with an antibacterial shampoo that contains chlorhexidine, benzoyl peroxide, or ethyl lactate. These shampoos can be found over the counter in pet/feed stores, online, or at Rudi's vet hospital.
4) Topical application of an antibiotic ointment, solution, or spray every 24 hours for the first 5-7 days of therapy may be helpful. The safe prescription mupirocin ointment is available through Rudi'svet and should be considered.
5) Surgical excision of excess skin folds is usually curative. The prognosis is good but lifelong topical maintenance therapy may be needed if surgical correction isn't performed. Practically speaking, such surgery is rarely performed in those areas.
The skin folds can end up affected by acanthosis nigricans. Dermatologists would slap my wrist for mentioning this as it's now considered a form of skin fold pyoderma. You might google the keywords "acanthosis nigricans" and see what I mean.
Please respond with additional questions or concerns if you wish.