This is a really positive result, considering, and I bet you are both relieved. I think the workplace is being really reasonable.
Going forward, honestly, as someone who's been through this due to disability, and had far, far less time off work than your DH, and my work were nowhere near as reasonable, your DH does need to be careful and make some effort to find solutions, as it won't be able to continue as is.
If the workplace are putting in reasonable adjustments as recommended by OH, then in future, they can dismiss him on health grounds. The process of dismissal after a warning can move surprisingly fast if the workplace want it to, so it's really important he takes this seriously going forward.
When I say this, I mean, he really needs to be doing some medical investigation into his health issues (more along the lines of the immune system issues), and, really consider reducing his hours, even on an agreed temporary basis, if he is off sick again between now and April. If, as an employee, you can show some reasonableness in your own managing health/sickness, it generally looks better, not just for the workplace, but should the worst happen in future.
I'd echo joining a union if possible. That being said, there is only so much they can do, and if he continues to have high levels of sickness, workplace can and do take action. I'd also agree that the process should be on capability grounds, however, it doesn't mean workplaces do everything they should, even very 'professional' workplaces.
He has my sympathies as someone with a disability and trying to stay in work, its really difficult. His work sound really good to be fair.