Sleep training ruined by sickness :(

We sleep trained our 9 months old about a month ago. It took about a week but it was going well! He would still protest after a week for about 5 minutes but would fall asleep and sleep through the whole night.

2 weeks ago, he became very sick and I wanted to comfort as much as possible because his cough was really bad. Even if he would cry a little, his dry would become dry and he would have a cough attack. So I stopped the sleep training and let him co sleep for about a week

Cut to now, he will not sleep without me. If I put him down, he immediately turns on his belly and hysterically cries. I’ve been pretty much making him fall asleep on me and then transferring but wakes up through the night and wants me to hold him.

I was under the impression that regressions or sickness don’t usually ruin sleep training once it’s been established but it really has for us. We’ve tried everything and he won’t stop crying.

What should we do now? :( so sad that all our hard work has gone to waste :/

He is in a 3/3/4 schedule