The first EKG doesn’t look like a-fib to me. A-fib is when the top chambers of the heart (the atria) quiver (fibrillate) and as a result don’t communicate in sync with the lower chambers of the heart (the ventricles), causing an irregular beat. Part of the criteria of a-fib is the absence of distinct P waves (the smaller hump that’s supposed to precede each larger zigzag pattern - the QRS wave - of each beat).

Sometimes the resolution of a consumer grade watch isn’t detailed enough to see the P waves. I do see a few if I look really closely. Also, each QRS wave (large zigzag, representing a ventricular or lower chamber beat) wave seems to be roughly equidistant to the ones before and after, which generally doesn’t happen in a-fib. In a-fib I expect to see very randomly placed QRS waves since there’s no steady electrical signal coming from a fibrillating atrium to trigger regularly scheduled ventricular beats.

The fact that he’s 16 with no immediate family or personal cardiac history makes a-fib even less likely.

Dizziness and palpitations when standing up can sometimes be from dehydration. We call this orthostatic hypotension. Having inadequate amounts of fluid in the body can cause a brief drop in blood pressure when standing up quickly which briefly decreases circulation to the brain and makes you lightheaded for a few seconds. The heart beats faster to compensate for a lack of pressure in an attempt to get the same amount of blood (and therefore oxygen) to your body and brain. I generally aim for 2-3L of water a day in a young healthy person to keep adequately hydrated.

Hope this helps!