Aero bars are a pain in the neck - are there strengthening exercises?
There are a number of things you can try.

Firstly the aero position on the bike and holding that position is often related to how flexible your hips and lower back are as well as the strength of your core, so yoga, stretches of those areas, and core exercises will make it easier to hold an aero position in turn taking strain off your neck.

There are some neck specific exercises that will improve neck strength specifically those similar to what f1 drivers use to handle the G forces through fast corners.

Take an exercise band and tie the two ends to a pole or bar at height so that if you were sat down it would be at approximately head height. This will essentially give you a loop. Sit down at distance away such that when you put the exercise band on to your forehead it is tight and offering resistance.

You can then pull your forehead forward to engage the muscles. There's a link to video if my description is terrible.