My husbands name is [name_m]Daniel[/name_m] and we have a [name_f]Felicity[/name_f] and [name_m]Desmond[/name_m]. I have another little boy on the way that will be less than two years younger than [name_m]Desmond[/name_m]. On my list of [name_m]Clyde[/name_m], [name_m]Ralph[/name_m], [name_u]Sullivan[/name_u], and [name_u]Dexter[/name_u] the last is the only one my husband likes. But I already get [name_m]Daniel[/name_m] and desmonds names mixed up. [name_m]Will[/name_m] adding another d name be too much. Also will any subsequent boys I have have to start with d?
Hmm. I tend to overly avoid repeating first letters, but that’s my hang-up and I don’t think it’s a universal naming sin. You’ve got [name_m]Daniel[/name_m] and [name_m]Desmond[/name_m], plus [name_f]Felicity[/name_f]. What does your first name start with? If something other than D, then maybe [name_u]Dexter[/name_u] is fine to add to your family. Then, I would say, if you have any future boys, I would 100% AVOID any further D names. Three is enough. 
Regarding mixing up their names, I feel like that would happen anyway. My kids’ names have no similar sounds, they’re opposite genders, and I am still constantly hollering out the wrong one’s name. I think that’s just a mom problem. Ha!
If [name_u]Dexter[/name_u] is your favourite, use it! I don’t think they’re too close for siblings. As for the pattern thing, you can choose to continue it or not if it comes to that. Too often I think people feel that they’ve “locked” themselves into a pattern somehow, but really you’re free to name each and every child whatever you like! “Patterns” are usually much less obvious to other people anyway.
Still, if you would rather avoid another D, how about: [name_m]Baxter[/name_m], [name_u]Rex[/name_u], [name_m]Wilder[/name_m], [name_m]Huxley[/name_m], [name_u]Rafe[/name_u], [name_m]Samson[/name_m], [name_m]Soren[/name_m]…?
Seconding what katinka said! I also just love [name_u]Dexter[/name_u]. 
Go for it!