When his son is taken by a monster into another dimension, a withdrawn divorcee must venture through the portal to bring him back.

Ever see House (1985) starring William Katt?? Because this idea is very similar.

To extend the Finding Nemo ?concept. Marlin was desperate to protect his only remaining child. I kept Nemo virtually caged to keep him safe. But this didn't help, he was still taken.

You also have a meaningless detail. He is a divorced. We don't need that. ?We don't get Amy free information from that.

?A man enters an alternate dimension to rescue his son from a monster that... but.... forcing him to ....?

Hope that helps.

A lot of the important details are in your logline, inciting incident, lead character, and goal.

My question would be, how quickly does the lead character go into the portal, is it on page ten of the script (I have no idea if this is written yet) or is the lead entering the portal the midpoint turn?

I only ask, because if the lead enters the other dimension early enough in the story, you might want to add a brief, (One or two words) description of what the world is like on the other side, Is it a dark fairytale world? Is it like Hell? Is it a world created out of old sneakers? Adding this quick detail mighty infuse a little spice to your logline.