What if DH and DS1 are blonde and DS2 happened to have brown hair. There are a plethera of differences children can have when compared to eachother. It is hardly psychologically damaging.
Apart from that, I find the whole "make him look like Daddy" thing completely perplexing. Surely you can point out that he's got Daddy's eyes, or chin, everyone can see that. Unless both father and son are going to be dropping their daks every time a conversation about family resemblance crops up, who's going to see it?