I am so sorry to hear about your darling Lab… I am sure your little Cavalier misses her dearly… Mourning in dogs is quite common and can take some dogs a long time to recover.
The first thing that is important to find out, is if there is anything specific about your Cavie that is concerning you, for example lethargy, decreased appetite, anxiety etc. This would be something to chat to your local vet about to make sure there is nothing else going on with him. We definitely want to sort any health issues out first before introducing another pooch into his life.
If he is otherwise medically well but needs more company, then possibly introducing a puppy would be a lovely idea. Saying that, introducing a puppy to an older dog’s life is a big change and can be quite stressful for your older mate. It is important to do this carefully so as to avoid any added anxiety for your Cavie.
A few tips for introducing a new puppy to your older dog include being present at all times in the early days when the pup and Cavie are interacting. You may need to stop the puppy from bothering your Cavie (with all that puppy love) at times. Exercising/playing with the pup before interactions can help keep the energy levels a little quieter for your older boy. Doing some training with the pup would be very helpful, and can be used as a tool to distract them at times if they are getting a bit too excitable for your older mate. Give lots of rewards (treats/pats/verbal praise) for positive interactions between them. I would avoid any punishments, just redirect them back to what they should be doing again. It is always best to use positive rewards to guide them to do the behaviour you want them to display.
Definitely have a chat to your local vet about your specific circumstances so they can give you more tailored advice as to what is going to work out best for you and your darling Cavie-boy. If you do end up getting a pup, be sure to send us in some photos of the two of them together!