Just adding my well wishes for little Brodie. Looks like Cathy and Kathy have given you plenty to consider/discuss with the vet for now. Sometimes a cold really is just a cold and has to run its course. I know it is natural to worry more over Brodie with his history and problems but I hope it is just a normal cold. Still, I know the challenge is to keep adequate nutrition going in. I think the constipation could have been the biggest factor in loss of appetite. No one wants to put more in when they are full up.
Hopefully, he will now take some tidbits. I would cater more during the cold -- definately consider green beans, pumpkin and bits of sweet potatoe. Not only do many dogs think of them as treats, they can help with the constipation. If there is a treat that Brodie likes and tolerates well, I would give it. You want calories right now and can worry about balanced nutrition once he is eating.