Motivation is the same regardless of what race/being they are. To make reasons for people who are unlikely to team up, team up... They need a common goal. Often in stories, we read someone who is suspicious tempt the MC into teaming up with them (LoTR with Frodo/Smeagol) because they both share what appears to be a common goal. It was unknown to Frodo that Smeagol actually was plotting to take it back.
It could be, in your case, that one god is just simply BORED, but secretly, through the advancement of your story, we find out that this child has meaning to the bored god that is unknown to reader/MC. As the saying goes, the best way to unite people is for them to have a common goal or enemy. Look at the real world... Our country wasn't in the best of shape prior to 9/11. Then everyone united together as 1 country, for a small duration at least, to fight the common enemy. Every time a terrorist attack happens, all the leaders meet to show support and unison in the fight, even if tomorrow they go back to arguing over politics.
Motivation could also very well be that he is simply bored!! He was bored with his mundane life as a god who sleeps all the time and saw a chance to do something different. Often times opposites attract. One guy who is the sun, one guy who is the moon (figuratively). The sun is full of energy while the moon is lethargic. Something about the sun makes the moon want to follow him on his quest.
We can again reference something from J.R.R. Tolkien with The Hobit. Bilbo was a guy who didn't want adventure... didn't want to deal with anything outside of his books and family treasures. Something inside of him made him want to go on an adventure with these sloppy, noisy Dwarves that were totally opposite of the life he was use to.
I suggest to do deeper character building if you are struggling to find a motivation. There may be a dimension you are missing that you need to put the pieces together. Through this reflection of the character bios, you may end up changing some things you realized didn't work, or adding in something that makes it all fall together.
Some times little to no motivation at all allows for the greatest motivation to be born. By the end of The Hobit, Bilbo was full set on helping them regain their home and understands their needs. Don't be so fast to throw all your cards up front. Let the story unfold and you may find that through this, you will discover a far deeper motivation.