I’m so sorry for your loss. StarlingWings has given you excellent advice. Budgies do mourn the loss of a close buddy, so it’s normal for them to be acting depressed at this time.
Usually within about a month or so, you should start seeing him move on and pick themselves up again. Some budgies will adjust to life fine on their own.
It’s hard not to feel sorry for them in this circumstance, but he will be okay.