Hi at 8 months you should really be doing just the PD part of PU/PD, does he sit up in the crib or pull up to standing. If so you need to not PU at all just do the PD part, in other words just lay him back down if sitting or if standing just sort of sweep his feet away whilst lowering his top half so you don't actually pick up. If hes not yet doing these things then that may be your problem because at his age being picked up will almost guarantee he will create like hell when you go to lay him back down. If this is the case could you just sort of sit him up with a hug then lay him back down.
I had similar problems with DD as she slept in a hammock and was physically unable to sit up in it until 12months when she had to be switched to a cot. I remember it being a very tricky time and also quickly became DD prop. I just had to go very slowly and use gradual withdrawal to get out of room. First I just laid my hand on her, not stroking or rubbing, then as she got close to sleep I would gently lift my hand and only lower it if she looked like she was going to wake right up again. Then I just sat beside hammock and used reassuring words. She did protest at this and every next stage but I just stuck with it. Next I stood beside hammock and slowly just got closer to door. By time I got to door I just went for it and left straight away and she was fine and just started going to sleep on her own again. All this probably took 2-3weeks but was so worth it in the end.
Hope that helps
