puppy constipated!!

We've been gradually changing our puppy's food from a secific dog food from one pet store that is hard to get (can't remember the name) to Purina puppy chow. He eats it fine and really enjoys it. We started giving it to me straight yesterday rather than mixing it with his food like we have been doing for the past 5 days gradually. Now he's finding it hard to poop. When he does poop, it is very constipated.
Any suggestions??
I don't want to go and pay high prices at the vet for this!
Should I just put him back on the other food??
TIA
JILL and my little shipoo PABLO :y_the_best:

 

Try giving him some plain pumpkin puree. The unseasoned kind. Maybe a tablespoon at breakfast and dinner. Its supposed to help with constipation and the runs. Did he have wet bowels before, if so he may just need to get his little bum used to pooing the way he's supposed to. Which is harder bowle movements.

 

For my kitten I used Mineral oil and it worked great within like 12 hours. I'm sure this would help. You can use your own discretion on how much to give.