Joint tenancy obligations for lad that has had to quit?

The Basic scenario is: HMO joint tenancy, five tenants. On a fixed element for first 12 months. Then break clause kicks in at one-month notice upon renewal of the next contract due July 22.

One poor lad (tenant) has had to quit university for his mental health, and he’s one of the joint parties. My understanding is that he (we? Including the agent) has two options.
1) Find an alternative tenant
2) Continue to pay the difference for the sake of the other tenants.

Needless to say, option 1 is the best and always open for the sake of the tenants. However, is he legally obliged to continue to pay the remaining the 7 months’ rent (I think so legally)?

What happens if he refuses?

Can hardly blame him if he goes down this route, it’s a lot of money, and I’m sure his guarantor won’t want to do this?

Would the onus be on the remaining tenants to pursue him for the outstanding rent, or myself as landlord and or the agent?

I had his mum on the phone in bits at the prospect of him paying £400 a month until July for nothing, effectively. I’ve sought legal advice from my conveyance, of course. Just wanted to test the water with you guys for insights of maybe the best ways to proceed.

I feel bad, and I am asking if this was your child, how would you play it?
Thx

Lloyd