Looks like sinus tachycardia to me, but I'd be interested if he's a steady 120 or has variability. Also if you're really unsure you can just give some adenosine and try to slow down the AV node to unveil flutter waves.
V1 P wave morphology is important
Typical Atrial Flutter (Common, or Type I Atrial Flutter)
Involves the IVC & tricuspid isthmus in the reentry circuit. Can be further classified based on the direction of the reentry circuit (anticlockwise or clockwise):
Commonest form of atrial flutter (90% of cases). Retrograde atrial conduction produces:
- Inverted flutter waves in leads II,III, aVF
- Positive flutter waves in V1 – may resemble upright P waves
This uncommon variant produces the opposite
- Positive flutter waves in leads II, III, aVF
- Broad, inverted flutter waves in V1"
Atrial flutter is a type of supraventricular tachycardia caused by a re-entry circuit within the right atrium