Am I understanding right that you send him up to do pjs/bath/teeth/potty/prayers on his own while you’re helping little ones?
We do things a bit differently, not a lot. My kids are 12, 8, 7, 5, 4, 2, 1, and 1 month. We have kids go upstairs to get pj’s on earlier in our evening than right at bedtime. With 8:30pm as bedtime we send them to put on pjs and pickup bedrooms at 7 or 7:30. Then it is downstairs with family for read alouds or scriptures, quiet activities, etc. At 8:15 everyone is told to go potty (only the youngers NEED the reminder, but we do have to get everyone in and out of 2 bathrooms)and meet at the couch for family prayer. Then one parent goes up with the older five (ages 4 up to 12) to tuck in and turn on the audio book in the hall. Kids also have booklights. The other parent stays downstairs with the 3 kids under age 3. Their bedtime routine follows (diapers, milk, snuggles, etc).
Our kids don’t get out of bed once they’ve gone up. It’s a severe breach if they do and they lose their booklight. They don’t get up to potty or get one last drink either as they had their chance 15 minutes before bedtime prayer. It simplifies things for us.
How to train them to stay in bed? Consistency! Always escort them back to their bed with no interaction beyond your phrase (Ours is “It’s bedtime. Stay in your bed.) and no extra drink, etc.
We do not begin this training until age 3, when we see they are ready to be motivated by being a ‘big kid’ upstairs with light privelages. Before that we have the child laying on the couch downstairs (age 2-3), or in our arms (age 0-1-2).