I'm assistant coach for my son's 4-5 year old team. If someone was acting that way, especially a man, I would NOT consider that being a critical parent. I would consider that being a bully.
PERHAPS I would jump the gun....I sometimes question myself because I have a strong physical reaction to a man that I feel crosses the line (my biological dad was very abusive to my mom before she left, and I think that even though I was very small, it made an impression).
I would walk up to him (with the other coach or my husband) and say "Listen, I know that you are trying to be an involved dad, and that you may have concerns for your son's feelings, but we are running drills, we are playing soccer, and we are trying to keep in mind that your son may be feeling stress with a new sport. But we are the coaches. You need to step back and remember that these are 4-5 year olds, and that you not only should not be coaching from the sideline, but if you have anything at all to say to my son you need to come to me, not him." If he can't handle that, then I would tell him "The rule on this team is that you are a dad, not a coach. You can cheer and encourage from the sideline, but there will be no snarky remarks. You will not be addressing my 4 year old boy. If this is something you can't handle, take your ball and go home, and we will have the association contact you". I absolutely would not be ok with a man being out of line with my 4 year old. This rule of course is different when we are at Kung Fu and the sifu there is talking to my son. Then, he is the responsible party and I sit quietly from the bench and observe. If I had a problem there, I would talk to the sifu in his office privately, and if it couldn't be resolved, I would remove my son from the class (something I don't see happening). I expect the same, I hold others to the same standards. (Though I really really appreciate any parent wanting to give positive input, assisting however they can, and cheering these little kids). If the association didn't back me up and money was an issue, I'd give him a refund and send them away. Am I wrong? Possibly. But I don't care.