Definitely a fan of [name]Maximus[/name].
You can’t get much more masculine than the name of the lead character in the movie “300”.
Sounds like you’re going with [name]Rex[/name] - which is a great name! I personally know a “[name]Rex[/name]” and I never once thought of canine during our friendship. 
Another alternative to [name]Max[/name] is [name]Magnus[/name].
Anyhow, middle names for [name]Rex[/name]…I really like jill’s “[name]Rex[/name] [name]Alexander[/name]” suggestion. It’s really going to depend on your last name (number of syllables, vowel/consonant sounds) to determine what flows well.
PS - I saw [name]Rex[/name] [name]Harrison[/name] in a stage production of Camelot when I was a very little girl He was fantastic! Wonderful name association.
I think [name]Rex[/name] is a strong, classic name. I keep meeting dogs named [name]Teddy[/name], which is my son’s name. Who cares? I find that people without kids love giving their dogs human names.
I think because it’s a family name you have even more incentive to use it. I wish I had a good family name to use – still trying to decide on our new baby’s name!
I think [name]Rex[/name] is a winner. As a child, I had a neighbor named [name]Rex[/name] that I adored. I found his name to be as interesting and as charming as we was! I might be a bit biased but I think that [name]Rex[/name] is a great choice! By the way, you have to go with what you want to name him! He is YOUR baby, you and his father should get to decide, not everyone on here or your friends/family.
I went to school with a boy named [name]Rex[/name]. People don’t make fun of … at least in my memory, just because people can come up with possibilities for teasing doesn’t mean that’s what they’re going to get teased about. I really like the name [name]Rex[/name] and don’t recall anyone teasing him about his unusual name. I think [name]Max[/name] might be overused, it’s still a decent name, and it is the most popular dog’s name, so where anyone figures [name]Rex[/name] is more doggie than [name]Max[/name], I don’t know, it’s not like Fido or Spot.
[name]Rex[/name] is very cool, a little different, but not so out there, doesn’t sound precious or pretentious, doesn’t need any extra syllables to have a nice texture or harmony. It goes nicely with [name]Gracie[/name].
I’m probably the most biased one here… my son is named [name]Rex[/name]. In all honesty when my husband first brought the name up I said “no way, it’s a dogs name.” But the more I said the name and thought about it I began to like it. Now that my little guy is almost 3 there is no way I could see him as the [name]Max[/name] or [name]Jack[/name] that I originally wanted. We paired his name with a traditional middle name as well, [name]Thomas[/name]. I love the name for all of the reasons mentioned here. It’s a great name for little boy, but works for a grown man. It is different without being too far out there or so different that it is now popular.
None of my family liked the name at first either, but now they are proud that their little guy is named [name]Rex[/name] and brag about what it means and how we picked it out.
what about rhett or rhys if not sure about or as alternative to rex?
think rex is fine if that’s what you like. not so sure about rex alexander though … something more simple might be better … like suggestion of rex thomas or … rex allen or rex michael or … lots to come up with …