Bummer! Steel bolt and aluminum head so the threads in the head are stripped, or nearly so.
If you have to do something and want to try simple first, use No. 1 Permatex, the kind the gets hard, and "glue" the bolt in place and vow never to remove again. Use just a little bit. Or use JB Weld, or re tap the hole for a larger bolt.
I am sure other suggestions are coming.