Buster sounds like he’s having a terrible time with his skin. Whilst itchy skin can be due to many causes, the fact that the areas affected on Buster are his feet and non haired underside which is closest to the ground, makes me very suspicious of a contact allergy. As you mentioned it could be grass but another common cause is Wandering Jew. This plant is very common in parks and in backyards so look out for this too.
Contact allergies can result in a vicious cycle. The skin overreacts to an allergen it contacts and becomes inflamed, the dog then scratches or licks the itch or resorts to dragging their body over carpet or even grass and this just exacerbates the problem! Once the skin is damaged from self-trauma, infection can set in and this causes further itching and self-trauma. Cortisone is great for short term use but can have side-effects when used long term.
Buster needs to see a veterinarian to break the cycle. They may prescribe an extended course of oral antibiotics to completely resolve any skin infection. Topical medicated shampoos can be used in conjunction with oral antibiotics but may not work on their own in advanced cases. His feet may also require a specific anti-fungal shampoo regime. Topical cortisone creams may be prescribed to apply to any new areas of redness when they first appear to prevent any self-trauma. Buster would also benefit from Omega Fatty Acid supplements to improve the health of his skin. Once the skin is back to its original condition it may be better equipped to deal with contact allergens.If Buster’s problem is indeed a contact allergy then keeping him away from the trigger, especially during treatment, is going to be the only way to prevent repeat flare ups. Obviously this is difficult if the cause is something as prolific and common as grass. You may have resort to keeping him confined to paved areas and not allowing him to lie on the grass or getting him to wear a T-shirt that protects his underside when he goes outside.