I had 3 blood test for serum cheatinine last one was 2.1. Had ultrasound. Dr. Sent me letter 6 months ago kidneys look good. Should I be concerned?

Yes. Serum creatinine correlates with msucle mass and above 1.6 mg/dl is abnormal for even a muscular man, unless he's elderly at which time some loss of kidney functions is expected. I'm glad you are being followed; we can be confident you have some mild kidney damage, but this happens to all of us as we age.
Slow kidneys. Although they look good on ultrasound, creatinine of 2.1 means your kidneys are definitely slow. Ultrasound rules out cancer, stones and blockages. But the slow kidney is called chronic kidney disease (ckd). It is recommended that you see a nephrologist who may or may not change things.. If your doctor is a nephrologist then you are okay.