Children have a lot of deep sleep in the first few hours. That explains your initial window of peace. He is then waking, and his expectation seems to be for comfort. This is fine, if not somewhat inconvenient for you.
At 18 months, many changes are happening in little developing brains. They are making more sense of the world during the day, and as their developing brain rewires its synaptic connections at night, they can wake in confusion with lucid notions of where they are. Furthermore, a child who falls asleep in ones' arms can feel as though they are waking up in a bathtub if they find themselves transferred to a cot, and panic - 'where the am I, where is mummy?!' Therefore, a close match on their 'going to sleep' and their 'sleeping place' is a good aspiration.
Have a think about triggers, is everything the same now as it was before he changed to this modus operandi, or as for many little people, has his routine been completely turned on it's head by the lockdown?!
You could try a few things, and take either the gentle steps or the hardball approach.
- Your target seems to be for him to be in his own bed.
- Your start point is that he is insistent on co-sleeping.
Gentle steps:
Could you re-jig your room so that his room so that his cot could be in a corner temporarily? You could then assess whether he is waking for genuine comfort, or because this has become a behavioural expectation.
If he insists on being in your bed, you could sidecar his cot. He is then next to you, but you will need to be strict about him being on his own mattress. You can then use this as a start point, and work towards the cot returning to his room.
No, the Cot to stay in his room? Make it more comforting and familiar with your scent, take a few fresh cot sheets and have them in your bed for a few nights, these can then be dressed into his cot to make it feel more like being next to mummy.
Night light? - I refer to earlier remarks about brain development, he could be panicking when he wakes because it is dark, or because the ceiling above feels a million miles away.
Hardball:
With your expectation of his staying in his bed, attend to his waking by tucking him back in and saying "night night, or sleep time" then leaving the room. You could adopt a transitional approach, where you stay in the room perhaps sitting quietly, but try not to respond to him
as your presence may be comfort enough.
Other thoughts for non-comfort possible triggers - room temperature, white noise, milk hunger. Just ideas...
Whatever you choose to do, even if none of the above, try to be consistent with the approach. Hang in there, and good luck with whatever works for you!
Sorry, I can't give an exponential suggestion here, work awaits.