history: Took my dog to the vet today.  They had to put him down.  I had to take my dog to the vet today Oh no_comma_ is there something wrong with your dog?
current: Yes, he hasn't been feeling well. The vet had to put him down. ;(
next: I know words don't really help in this situation, but I'm so sorry. I've lost many pets and it rips your heart out. I'm sure you're dog had a great life with you if it's any consolation.