Need a drill to help me keep my arms extended through my swing.

Buy the "Orange Whip"

It is a very good training device for warm up and tempo training.

A routine exercise would be - use a 5 iron.

With a slow motion, start with a pendulum motion like the grandfather clock swinging weights.

Back and forth slowly, increase travel distance progressively.

Key is to build a 3/4 swing back and forth evenly and controlled.

Key thought is maintaining arm extension through the swing with hands traveling towards your target.

You should feel your right hand pushing your left should upward just after imaginary impact position.

Final results would be a smooth full swing without hitching your left shoulder allowing the left elbow to bend until completion

of the swing.

If you do not achieve correction of your habit, maybe consider a lesson.

Club Rat

Or he could save the hundred plus on the whip and put it to use on lessons.. This might not save money, but will save the time of experimenting on his own?

That's funny because Mark Crossfield just did a video on this very thing probably less than 2 weeks ago.  Basically he said.. what does it matter what your arms do after you hit the ball.  The ball is gone.

Is he right?  I dunno, but he's certainly better at the game than me and his point seems pretty straightforward and logical.

So then the OP is probably barking up the wrong tree anyway.

I thought I lived in a chicken coup I used to chicken wing so much ... but have found the towel under the arm trick seems to help in practice.  I think the term or thought  is "stay connected"

Didn't realize I did it so much until I look at photos of my swing ...

I guess mentally I would prefer to have a standard movement through the ball after contact.  In a physical sense, the ball is released from your club I guess it doesn't matter what you do with the club, but as a previous user said your downswing will have a huge effect on how your follow-through is maintained