I have a DD who's just turned 3 and a 4.5 month DS, and DH often not back before bedtime. Here's what we now do - we've been working on it for a few months and whilst it's not always perfect, it seems to be the calmest way for us to get through the evening routine... (NB for any chance of success I still have to prepare everything - pyjamas, drink, bath stuff, baby changing stuff, box of toys etc etc about mid afternoon - so by 6.00 everything's ready - apparently I am not a natural at this!)
6.00 both get undressed (or popped in the bath if it's bath night)
6.15 DD goes into our bedroom where I've made her a little corner (it's space that is just hers for when her brother is asleep in their room), where I've put a box of toys - I have about 5 activities on rotation - lego, happyland, doll's house - and she plays quietly, chatting to herself about the day, while I feed DS, read him a little story, sing and put him down.
6.45 I join DD, chat about what she's playing, read story, Bible, pray, sing, then sneak into their bedroom with her and tuck her in.
I know that DD loves having some one-on-one time before bed, and she looks forward to seeing what's in the box to play with that evening, so it works well for us. It's also a great time for her to wind down and process the day. It seems also to have been good for her to learn to wait during DS's bedtime, and to be rewarded with time alone with me once he's asleep. Would your DS play alone for a bit? You could gradually increase the time maybe, starting with just a few minutes as he gets used to it?