He is not too old for sleep training. It's just the work you need to do through the 18 month sleep regression (which this sounds liks) is about actively teaching him stiff, rather than just tolerating crying until a younger baby accepts your way. This time you need to teach him through repetativness, consistency and clear expectations.
A big mistake made here is getting rid if the cot, it will make everything from here going forward mire difficult. You will most likely end up cosleeping in his bed.
So start with bringing back the cot.
At this age you need to give your child autonomy over their movements, so indeiendance if choice. But (big but) set your expectations and accept nothing less. So you are accepting that baby can (for example) climb up the cot / stand up, but you expect them to choose not to at sleep time. Largely this is a behaviour issue, not a sleep issue. Welcome To Toddlerhood!
So your expectations need to be that at sleep time baby will lie down, be still and be quiet. What you can't do is make him sleep. But you can expect him to lie down, be still and be quiet (and he should hopefully relax his mind and body and so sleep)
Start with independence of movement: always put him in the cot standing, tap cot mattress and tell him to lie down.
Never put him in lying down or even sitting up. Play instruction following games in the daytime to encourage this. You'll be surprised how much he understands and can follow instructions. Plus he will love the praise. Take this to the bin, put this on the table, give this to daddy, where is the ball, bring me the tv remote. Give as much independant movement as possible too. Don't lift him into the sofa, tap, tell him to get up, watch him climb. Plus anything else physical he can do.
With instruction following and physically following instructions sorted, your bedtime wants to be:
● Into cot stood up in a sleeping bag.
● Leave dummy on cot matress. Tell him to get his dummy. (Don't give it him). This will get him sitting down.
● Tap matress, tell him to lie down. Wait. Praise, smile and bend down to him in the cot when he does.
● Say a mantra, something that encompasses all your expectations. "Sleep time now. We lie down quietly to go to sleep. Nan night", for example.
● Stay bending into the cot. Eye contact and smiles as you say mantra. Place your hand firmly on his chest and wait. Either a few seconds or a few minutes until he is still, quiet and calm.
○ If he doesn't get to being still, quiet, calm - remove eye contact, have a cross face, swiftly put your hands under his armpits and lift him straight out of the cot to stand up in front of you. Kneel to his level. Say "No", have a cross tone. Repeat mantra. Then start again. Keep on repeating this over and over and over again until he does get to being still, quiet and calm with your hand on his chest.
● Within a minute or so of being calm, quiet and still (So don't linger) remove your hand but stay bent over the cot. Eye contact, smile, positive and compassionate face. You need to reaffirm your praise in your facial expression, while being quiet and calm.
● Stay bent over the cot for a minute or two of quiet, calm, still. Then stand, turn and face the door. Stay stood by the cot but facing away. Have something to do so that he sees he is being ignored but you are there. Scrolling on your phone is ideal. Stay until fully asleep then seek out.
○ If at any point he goes for sitting up or making noise, turn back to him, bend back over the cot. Repeat mantra, tap mattress and tell him to lie down, hand in chest and start again. Just keep repeating over and over and over and over and over again, always the same.
● Once he's OK with you scrolling in your phone next to the cot while he goes yo sleep, give it a few days and start taking a step or two away from the bed to wait. Then after a week or so start waiting by the doorway, then outside the door, then in hallway just out of sight.
○ Throughout all of this at every step, any deviation from your expectations (lie down, be still, be quiet) you go back and start again. Over and over again, always the same, always consistant.