Hi J..
First let me say our household is probably a-typical in that dinner time at our house isn't the same every day (neither the time nor the participants), and we're okay with it. My kids have grown up this way, and it is "normal" for them. My husband works odd hours and is never home at the same time every day. In fact, 2 days a week, he doesn't get home until our kids are long since in bed (I'm talking 11:30 pm). So, we just accommodate the best we can. When he is home by 6:30, we usually eat together. When he is not (he eats dinner at work), the kids eat at a "normal" time without him. But due to after school activities, sometimes they are eating at 7:00 or 7:30. It just all depends. My kids are older (8 & 11) but it has ALWAYS been like this due to my husband's job.
So, realize that "normal" dinnertime, is what is normal for your household, and don't set up some "June Cleaver" example of perfection as the scale you use to grade yourself.
When our kids were little, before they were school age, I tried to make "lunches" more of a family mealtime on days when my husband would not be home for dinner. Maybe if you want to have "family" mealtime, you can make it lunch or breakfast with you and baby. Or just incorporate it into weekend meals, when everyone can participate. The rest of the time, do whatever works for you. Lots of people intentionally feed their kids first, so that they can enjoy their meal with their husband, without the kids making it a hassle. Do you have a warming drawer in your stove? They are great for keeping things just right, without that "reheated" or "warmed over" feel. Or make a meal in a crockpot. Those are "fresh" whenever you serve it...
Just don't set yourself up to be constantly judging yourself over this issue. In my experience, having a "groove" with only one child and me at home was next to impossible. It wasn't until we had #2 that you could call anything a "groove". When it is just the 2 of you, flexibility is the name of the game... take advantage of it.
If you want to push back his dinner, you could offer some healthy snacks to tide him over.. but then you will be wreaking havoc with bedtime. So unless you plan to alter bedtime too (never a good idea if what you are doing is working well) then I would just feed him first and be done with it. If you want him to sit with you and your husband, that's okay, but he doesn't NEED to eat to enjoy your company. If you choose to feed him a 2nd meal that's okay too. When he is older and can wait later for dinner, then he will merge into the routine without you having to explain that "we only eat dinner once". But, unless your husband can get home earlier to eat, then you won't be having weeknight family dinners for a while... it's not until kids are closer to 7, 8 and 9 years that they can stay up late enough to wait until 7:00 to eat. They simply need to go to bed/sleep too soon after to be eating that late, imho.
Oh, one other idea: If you are looking for ideas that will "keep" from 5:15 until hubby gets home for dinner... try out a salad. Kids love the veggies: cucumber, red bell pepper, black olives, romaine, carrot sticks (with ranch to dip?) and you can put cut up chicken breasts or whatever right on top. My husband loves salads like that for dinner.