When I first started quilting I used an old fasioned seam guage to consistently check my seam allowance. You can get a seam guage, seam ripper and thread snips for $2.00
I also laid down several rows of masking tape on my old machine bed to help feed the fabric in at a consistent quarter inch. When I got my Bernina I laid out the tape on my table.
When I first started, I would measure each seam with the seam guage to ensure I was maintaining a 1/4" until I got really good at sewing a 1/4" My machine didn't have a 1/4" presser foot so I took an ultra fine sharpie and put a little mark on the presser foot where the 1/4" seam allowance would be.
The biggest thing is where your eyes are focused as you feed the fabric in. You may want to ask DH where he is looking. Is he looking at where the needle is sewing or is he watching the edge of his fabric to ensure it is lining up with the proper place on the presser foot? Tell him it is like pool (got to speak in man terms, donchaknow :lol: ). Just like in pool, when you take your shot you want to be looking at the pocket you want the ball to go in not the cue ball. So with sewing he wants to be looking at where the fabric is lining up as it goes under the presser foot, not the needle.