Is this an abcess?

An outside cat that has been healthy up until now 2 weeks ago started bending his ear and walking with his head tilted. I'm unable to handle him or look in his ear. Now I see this behind the ear, could it be an abcess, or scratching from possible ear infection or ear mites?

Only a vet could find out for sure... But I'm not sure how you could get him to the vet without being able to handle him.

If it's a ruptured abscess, then it should have the appearance of a hole or pit. I can't really tell from the picture if that's the case. Hot spots from fleas/scratching do look similar with a patch of hair missing and a raw area. However, if it's like a little hole, I'd say probably abscess. Abscesses can be bad, you should probably figure out a way to get that cat in to see a vet.

As it started 2 weeks ago, and is now a hairless spot, hair loss due to follicle death after infection and stretching of that skin is possible. Ear infection leading to self trauma usually involves scratching with the hind feet, and he's have little wounds (excoriations) from scratching around the inner and outer pinna (the thin skin/cartilage held erect from his ear canals) and often at the cheek also, as their canals go down, then into the skull towards the brain, L-shaped path. If the back aspect of the pinna/ear-base was all that was affected, less likely self-inflicted.

Regardless, it looks like he lost hair, and has an open wound centrally in the area of hair loss. The skin is pink, healthy looking, not dark or purple, an indicator of tissue death or bruising..

An exam that can appreciate not only the appearance up-close, but probing around and in the wound will help accurately diagnose and treat the cause of  the alopecia and wound.

If scratching a lot, ask your vet to trim his nails while they have him,  which will be more blunt after he walks around for a day on freshly trimmed nails, and he won't be able to claw himself as bad while treating his wound for a few days.