I want my sim to in the future have a family with his imaginary friend :)

I already turned him real

But my problem is.. they're both male.

Can I somehow turn the imaginary friend female? Is there like a gender change potion in the game or something?

Sexy gay imaginary sim lovin' and an adoption?

If you want to turn the friend female you'll have to edit it in CAS... turn testingcheatsenabled on, right-click on it and select 'edit sim in CAS'.
You should be able to switch the gender in there, but I dunno whether they'll keep their weird clothes or hair.

Nothing happens when I right click him, and when I shift-click my only options are modify traits, trigger age transition and favorite music.

You know you can have males get pregnant, right? If you're comfortable with mods (I don't know why people wouldn't be with just one single mod) you can get male pregnancy in your game.

^What Erry said, and I'm not sure that Gender changes stick in CAS. I just know that weight and muscle changes don't stick.

I was able to get males pregnant, untill the whole fiasco here:
viewtopic.php?f=36&t=37961
And here:
viewtopic.php?f=36&t=37985

And I can't re-instal it because me and my friend share and exchange them, I bought a few sims games, she bought a few, and we lend them to each other.
Only one we both got was pets.

I refuse to go ahead and spend over 100 bucks to also get the expansions she's got for a game that's this ridiculously bugged. That and my pc needs an upgrade, I'm lucky I'm experiencing this little lag as it is.

(in short all .package mods don't work for me. only sims3pack which I put in the downloads folder work I think)