Don't kick him out. Support him. He's messed up badly but he won't resolve it without help. Explain to him that this is solvable.
First, I'd contact his uni. Extra grants are often available for students with parents on very low income. If he hasn't claimed any, get him to apply and ask if they can be back dated. Explain the financial mess he is in as a result and the pressure it is putting on his grandfather. Ask about any emergency hardship funds too.
Also ask uni for emergency counselling and explain how low he is feeling. Help him contact his tutors to explain his mental health and ask for an extension or the explore possibilities for resitting a year.
If he doesn't already have one, take him out to look for a job in a cafe or restaurant. Restaurants are great as they often feed you, so he doesn't have to pay for main meals. If he doesn't feel up to being a waiter, he can at least be a pot washer. Get him to work at least four evening shifts a week and have the money paid directly to his landlord to pay off the arrears.
Get him to phone his grandpa and promise he is dealing with it and that he won't be paying for the rent (if he can't afford it) or if he must and can afford it, that your son is immediately paying him back with interest from his job.
Reassure him that no one in the world gets through life without making some massive mistakes, and at least he's had the good sense to make his early one.
I don't mean to be soft on him, but I feel so deeply for his generation, going through uni during lockdown. It's been absolutely shit. No casual jobs for two years, no social life, no face to face seminars. He needs to take responsibility but he also needs to see that circumstances have been far from ideal.