Mostly it's a phase you will need to ride out, it's such a new experience for him that he can't help but want to do it all the time and as you've said yourself sometimes when they get up they can't get back down.
All though it's a case of riding it out you could try:
- lots of floor time during A, don't use any bouncers and limit how long he is in other restraining things such as high chair, car seat, push chair etc, the more time he gets on the floor the better, he'll get his skill and move on.
- A longer A time. Although he might not really need a longer A time it could help with getting him really tired before he's put into the cot, so he is less inclined to stay awake and stand up. Some floor time prior to nap time could help too so he gets his body tired as well as his mind (physical tiredness can be very important to some LOs and at certain phases)
- teaching him to sit back down. Tracy said in her BW book, to take LOs hands on the bars of the cot and one by one slowly bring them down so LO is holding the bars but lower and lower, eventually his bum will plop onto the mattress and from a sitting position he is more likely to be able to lay himself down. Many LOs work out how to stand up or pull themselves up and get stuck.
- Use 2 different phrases when helping him, 1st "sit down" 2nd "lie down" (followed with your key phrase, "go to sleep" etc), it may be too much to work out the "lie down" instruction when still standing. It made a difference to my DS.
- keep reminding yourself it's not for ever...as tiring as it is it will likely be for a short period.