Hello & welcome, I am Dr. B, a licensed veterinarian & I would like to help you with your wee one today.
Now as I am sure you can appreciate, coughing in the older dog can be caused by a range of conditions. This includes bacterial infections and heart disease but also viruses, parasites (lungworm, heartworm), asthma, fungal infections, bronchitis, and tumors.
With all this in mind and the fact that he has had intermittent signs (which make heart issues less likely), we'd not want to risk harm by just giving a diuretic inappropriately. (Especially since this could cause urinary accidents, dehydration and circulatory collapse if used on a dog that doesn't need it). Instead, if Rowdy has not responded to antibiotics, then we really need to think about having his vet xray his chest. This will be the most straightforward way to start narrowing those above concerns down. It will let your vet check his lungs and upper airway for disease and at the same time it will let them look at heart size. If the heart is found to be an issue, then Lasix may be an option but usually isn't the first treatment we'd use (since it just removes the fluid build up secondary to heart issues and doesn't actually help the heart to prevent that in the first place).
Overall, this doesn't necessarily sound like congestive heart failure, especially if it has come and gone in the past (since heart issues are for life). Therefore, it would not be a good idea to give Lasix here. Instead, we really need to think about an xray to see what is in his throat or lungs. And based on that, we can treat this properly and get him settled.
Please take care,