There seem to be two very separate issues here. How you fit in TTC, and your DSS's bedtime routine. I think it would help solve them if you can separate them.
I can't really help with the TTC one, as I haven't been through your experience. I can see why you'd want to take a more relaxed approach when possible but not why that rules out quickies first thing, at lunchtime, or whenever it fits in, too.
The bedtime issue though is a problem for the sake of DSS's health and development and should have been tackled far earlier, in collaboration with his mother. It's close to being too late now, as he approaches the teenage years, their hormonal chaos and social pressures.
He needs a good routine, so that he can rely on getting enough sleep and being rested, healthy (mentally and physically) and ready to face the world on good terms each day, especially when many things that happen during those days will present new challenges and change around him. Someone overtired and unable to recover from tiredness quickly, through a good night's sleep the next night, can easily spiral downwards into emotional problems and inability to concentrate on school work and to move forwards in his life.
I honestly think that good habits and self-care routines are one of the best gifts a parent can give their child. They support lifelong health and development and just cut out a huge number of trivial things that can otherwise take up valuable mental space, far better used for more interesting and rewarding pursuits. I think your DH has been very self-indulgent about this, to his son's detriment.
How to tackle that now? Through modelling good behaviour and gently, circumspectly, enforcing it I think, so that it doesn't feel like 'enforcement'. Rather, doing fun, stimulating things together earlier, making sure everyone gets enough exercise in enjoyable ways, naturally winding down in the later evening - and avoiding screens right before bedtime. There's plenty of literature on the detrimental effects on sleep of screen light at night. Watching a film together, then getting ready for bed is one thing, leaving the boy alone in front of a computer at what should be bedtime is quite another.
He's going to turn into the kind of teen who's up into the early hours seeking emotional stimulation and human interaction online, to make up for what he doesn't get in real life during the day. That's a self-reinforcing fedback loop, to the detriment of education and human relationships.
I'm not saying that all teenage use of the internet is bad, by any means. Or all late nights. I am saying that if these things don't sit within a context of healthy family and peer group relationships and an active 'real' life, with built in good habits, that it can provide a very unhealthy and detrimental channel for teenage emotional development.