Poor Ollie :( Sounds like he sure had a rough go of it at the new groomer. I am very glad to see that you are going to have a vet check him over...that is definitely the first step just to be on the safe side.
Should that come out all clear, the next step is rebuilding Ollie's confidence. I would suggest short walks around your garden/yard with plenty of yummy safe treats, like small bits of boiled chicken breast (no skin, no bones, no seasonings) and with a cheery voice, good boy Ollie, that's a good boy and when he's happy, treat him. Then move to the end of the drive way, then a little way down the street, all doing the same routine. Take it very slowly with Ollie...going too fast will back fire on you. Do this over a period of a week at minimum. If he starts to get uncomfortable, go back to where he is calm again.
As time goes on, you can travel further and further. Do NOT tell him its ok in a soothing voice when he is nervous. This will just reinforce the negative behavior. Just ignore it and go back...on your terms though, not on his.
Since he's a younger dog, it will take some time but I think Ollie will do just fine and again, I can't stress taking this slowly enough. :)
I would also call the groomer and see how he behaved there. They may lie...they may tell you he was nervous in the cage but do ask about his behavior. I would also consider a mobile groomer if you can find one, for now..until Ollie becomes Ollie again.
Do pop in and let me know how its going with him and if you have further questions, please feel free to use the reply box or click the reply tab to reply to me.
Thanks so much and good luck with your boy!!!