Although it may seem like a good idea, it’s best to try communicating with your eevee to see if that’s what he would like to do. If you have an espeon or know anyone with one, it would be an excellent idea to have your eevee meet them in order to shadow them, in a way. That is, exposing your eevee to an espeon will help it decide whether or not being one is right for it. If he’s sure after interacting with an espeon that he wants to be one, then by all means, proceed.

If he seems hesitant, however, consider other options. In truth, all of eevee’s evolutions have unique ways of navigating the world that isn’t necessarily reliant on eyesight. Jolteon, for example, read electrical charges in the air. Vaporeon does the same with humidity, but leafeon can detect changes in the ground through its paws (likely due to its connection with the earth, as a grass-type).

Even then (and in response to your other question), there are certainly ways a trainer can assist a pokémon with an impaired sense. Training them to rely on their hearing and smell, for example. Designing strategies that take your eevee’s disability into account while leaning on what he can do instead, for another. Some trainers even use the impaired sense to their advantage, as most opponents will either underestimate a disabled pokémon or not expect one at all, which means they won’t expect whatever strategy you throw at them. Blindness can even throw a wrench into strategies that rely on a target being sighted, including and especially the popular Attract strategy, as well as the somewhat rarer Hypnosis.

The point is that while evolving your eevee into espeon could help, it should ultimately be up to him. If he chooses not to evolve into an espeon, rest assured you still have plenty of options. You just have to think creatively about what he can do instead and build training techniques around that.

Best of luck!