Good backstory and you have fleshed out the characters well. My only critique would be that there is a lot of telling, not showing here, but showing can be difficult with backstory...though I thought the scene in the backyard where Justin's dad tries to hit his mum would be the perfect opportunity to show, not tell.
I was in the dark for a long time about exactly HOW to show and not tell, until the wonderful memoirist Patti Miller told me there are three basic ways: 1. dialogue (yours so far, is very good!) 2. action 3. sensory perception eg: SHOW us how Craig's heartbeat raced or how he got hard at the mere thought of Justin, instead of telling us that he thought that giving J his number went well, just as a random example.
Looking forward to the next chapter!