Yes his behavior has been different during the day. A bit fresh kicking and hitting, slamming doors etc
This says he's picking up on the changes with you being pregnant & feeling like he's not getting his needs met in the way that fills his cup. He's on the young side, but have you any idea of his Love Language, because it kind of sounds like physical contact may be part of it.
Also with the new baby coming along he may not be able to totally comprehend it all but already there is the bump that is the barrier to his closeness to you & his hitting & slamming doors shows he's "stressed" & that means at the end of the day he's carrying more emotions & that is always harder to get to sleep with.
Now rather than thinking about him as a toddler who you don't want to develop bad habits with, what if it was a situation with a child who was "sick" & was struggling to get to sleep or it was a friend who'd been having a hard time in their life... would you see their calls for your attention as a 'bad habit' - societies education of us has double standard, children who call us back at bedtime when they are physically healthy are manipulative, but if they are sick they are ok... but even as 2yo's we are already discounting their 'mental' health & saying it is a bad habit, when the reality it is communication of their needs.
I totally get the "what happens when the baby comes I can't spend that time with him then" but this is now he is asking for your help & from a BTDT, when the baby cries & we go to them, but we leave the toddler because they are older we put a wedge in the relationship because we say (to the one who is more likely to remember) that the baby is more important & that's only really because dealing with a crying baby is about dealing with a physical need & we've all been educated to see emotional needs as less important, but are they really?
In 4 weeks time I have 22 hours of flying... the difference between me being relaxed on that flight is an Aisle seat... until that is confirmed (it's part of a group booking so it's a request but not confirmed) it's worrying me... I know mentally that really if I sit between 2 people it will be ok, but it's also a phobia & I'm struggling with it... putting me on a plane in seat A or B won't make me realise it's a bad habit that I don't like not being on the aisle, it will still make me sweat & shake, I can do it on short haul for 2-3 hours when I'm next to my own children BUT next to complete strangers it becomes different... it's kind of the same with your son he may not be able to cope with bedtime (SA or learning fears) on his own but when you are there he can.
Also solving it now doesn't mean it won't come back when there is a newborn... I remember sitting feeding DS2 with my hand on DS1's leg when he napped because DS1 wouldn't settle & that worked & I think it helped him feel I was still there for him 100% even though there was a new baby taking my time up... he saw Mum could look after both his needs & his brother's too.