What should I do? Unable to merge forks due to a mistake.

I’m contributing to another user’s repository. First I forked his repo, changed some files and made a pull request. He merged it and made a few more commits. I tried to merge them into my fork, but by mistake chose to squash them, so they were registered as my own commit. So now I can’t auto-merge (and neither can he, apparently), and all merges must be done manually. Is there any way to fix this?

Can I fix this by deleting my repo and forking again?

You should be able to make merge-able pull requests by adding the other user’s repo as a remote, then deleting your local default branch (presumably master) and checking out a fresh copy from the other repo, then basing your new pull request(s) on the clean default branch.