stillnoname:
He hasn't said anything about it but I mean I'm sure he notices. How do I bring up the topic? Do guys even notice when their SO Gains
At your height, I doubt a guy would notice until you gained about 5 pounds. And he might not say anything (out of politeness, his own insecurities, etc.) until you gained 10 or 15. And if the gain is slow, he might never say anything no matter how much you gain.
You're going to have to prompt him, perhaps looking in the mirror in front of him, poking your belly out a little and saying, "I think I'm getting kinda chubby. Can you tell?"
That could start the conversation you desire, and you will know how he feels.
Best of luck.