1. (Red) — Spike and dad have the same upper scales. 2. (Green) — Spike and Smolder share the same knees and feet. 3. (Light Blue) — They all have the color of their stomach that go all the way up to their jaws. 4. (Purple) — Spike and Smolder have similar ears, only difference is that Smolder has horns going out (possibly taking after their mother if she has them). 5. (Navy Blue) — Spike and Smolder share the same stomach pattern and are both a little chunky. 6. (Burgundy) — Smolder and dad share the same eye pupil, and their eye colors are very close! But knowing that eye color sometimes fade when people get older, dad could've had that color once!
We can't see the rest of Dad's body yet, so the rest of the resemblance would have to wait until his debut.
Also to support the claim that Spike and Smolder COULD have a familial relationship: https://twitter.com/TheBiggestJim/status/978034625007136768