Standing up in crib

Anyone else struggling with their LO practicing pulling themselves up to standing in their crib? Every time we lay him down for a nap or bed he keeps trying to stand up instead of going to to sleep. We’ve already done sleep training and it had worked beautifully til recently. Now it seems like we’ve got to go back to rocking him to sleep or he gets overtired. Any advice or is this just a phase we’ve got to deal with? I’ve already lowered the crib mattress as far down as it will go and I’ve tried keeping him awake a little longer so he might go to sleep as soon as he’s laid down but nothing seems to work. Help!!!