After you go on once - after BT - do you keep going on and out? I'd be tempted to be very firm but calm with him and set out your expectations. I did this all day long too, and reminded Z during bath, books, extra wee etc.
Something like this worked for me.
'Its BT now Z. Remember you need to go to sleep now so you can feel rested for a fun day tomorrow. You have your water if you need it (offer it now too) and your night light. You know I will always come if you need me, but if you call me and you don't need me I will note happy as I need to get myself ready for bed too.'
If he gets out of bed, march him right back to bed, 'it's BT, you need to get back into bed and go to sleep. goodnight'
I'd lay out your expectations for nights too. Clear his room at BT of anything he may find 'scary.' Talk about how you will feel very proud of him when he sleeps all night long tonight. That you will come if he needs you - bad dream etc - but he needs to get his rest and sleep till morning. Let him know you need your sleep too.
It is boundary testing, but some of it is probably fear/anxieties etc. so firm but gentle works well here.
No napping am I right? - when was he up. I'd stick to a 12 hr day/night but BT no earlier than 6am