If he wanted to, he could probably sue Tesla for tortious interference of his previous job, since he gave notice at the previous job because of them. Also, it really sucks that Tesla didn't offer him severance given that he was effectively laid off. Hopefully he can at least get unemployment.
He would lose 99.999% of the time thanks to `at will` employment.
The central issue is that Tesla gave him a job offer and based on that offer he resigned from his prior job. Tesla then reneged on their offer weeks later. The damages are the lost wages due to them not following through with the job offer after they caused him to resign.