Now is the time for teaching your little one to follow instructions and developing gross motor skills (bear with me, this is relevant to your sleep issue)
You will be surprised how much your 8 month old understands, even without the verbal skills to talk.
Start with games in the daytime. See what he understands. Things like "where's your nose/lips/hands etc" Also play "stand up, sit down" and similar games - just tell him (as a game) to stand up, clap when he follows your instruction. Tell him to sit down, lie down, roll over etc. All the Simon Says type instructions, but without the "Simon Says..." bit at the start (which would be quite complex strategy for an 8 month old to understand). Babies and toddlers are keen to please so you shouldn't be getting any sullenness - if he is physically able to and understands your instruction, he should follow it as long as you make it fun with smiles and praise.
You want to get to the stage where
(a) he physically can go from standing to sitting to lying down without any help
(b) he understand what you want him to do when you say "lie down"
(c) he knows how to follow simple instructions
From that start point, completely stop lying him down in the cot. Always, every time you put him, put him in standing up. Tap the mattress and tell him to "lie down" - have him do it all himself without help. Similarly, if you use a dummy or cuddly toy, dont hand him this. Throw it into the cot and tell him "where's your dummy" (or whatever) so he finds and gets it independently.
As mentioned, little toddlers are almost universally eager to show how clever they are in following instructions (as long as they understand and can do it). So while you will probably have to keep repeatedly saying "lie down" every time he gets up in the cot, it is teachable to get through this phase. You just have to be persistent and patient about it,