It’s incredibly important to bond with your pikachu as much as possible. Don’t be smothering or overprotective, of course, but be there for him. If he’ll allow you to do so, pet him and hold him. Some grieving pokémon take especially well to physical contact, as it makes your bond with them literal: you’re essentially telling them that you’re there and that they aren’t alone. To this end, yes, you’ll need to spend much more time with your pikachu, even if he doesn’t seem to respond to you at first. Even just an extra half an hour a day will be of immense help to him.
Also, don’t be afraid to use a little positive reinforcement as well. If your pikachu likes a particular type of toy, buy more of that toy and give him one whenever eats. Show him affection for coming close to his food bowl or water bottle, and whenever he responds to you in a positive manner, give him a bite of his favorite types of treats.
Toys are also excellent distractions, as odd as this may sound. It’s simply that giving your pikachu a means to keep his mind occupied whenever you’re unable to sit with him helps him avoid dwelling on how much he misses his father, which in turn will allow his mind to heal. It’s very much like how humans sometimes find ways of distracting themselves when they’re faced with grief so they don’t dwell on their hurt and allow it to dominate their psyches.
Speaking of, try to enforce your pikachu’s daily routine as much as possible. Like toys, routines are excellent distractions, and they establish a sense of normalcy in a pokémon’s life. If your pikachu is fed at a certain time every day, be sure to feed him at that time. If he trains at another time, don’t let that hour go by without training.
Most importantly, remember that pokémon are very sensitive to their trainers’ emotions. If you’re strong, calm, and patient for and with your pikachu, this will help him remain calm and open up to you. Yes, this means the grieving process will take a lot of effort on your part, but you need to show you care and that your pikachu can rely on you when he needs you. So have patience and do everything you can to remind him that you love him and that he’s not alone.
Best of luck, anonymous.