and which tree is it?
If it's something rubbish like Ash, I'd let the clematis go wild.
Your problem relates to mine (http://www.gardenersworld.com/forum/problem-solving/holly-tree-cematis-montana-and-bird-cover/909374.html) , except I know what the tree is. My plan of action is to remove all the clematis stems throughout the tree (done), cut back the clematis almost to ground level (I know it will survive as it came back from what looked like dead wood last year) and attempt to train the clematis along the fence instead. Whether I succeed remains to be seen, but what I do know is that the rampant nature of the clematis severely distorted the holly, and the matted winter stems looked horrendous. It's not a big tree though, your son's may cope better.
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I have a climatis growing through a young Amelanchier Lamarckii (June berry) tree, I just leave it to do its thing. You can make out the blue clematis to the left of this photo
With the blue on, I just leave it alone as I forget its there when not in flower. My other clematis I cut them back at the relevant time of year, they all have different pruning times, largely dependant on when they flower. Ive got a bit of a 'thing' for clematis, they are one of my favourite plants and I have quite a few of them in the garden..
Heres a better photo of the blue (unknown) clematis growing through the tree..
I have Clematis Dr Ruppel growing over an archway (photo taken when not in flower!)
Dr Ruppel in flower
Clematis Nelly Mosser crawling along the ground in my border (which I think they do in nature), eventually climbing up the obelisk..
Clematis Pillu growing through the Honeysuckle and up my garage wall..
Clematis montana almost completely swamping my garage..
Clematis Bees Jubilee in a planter against my garage wall..
As you can see, I kind of like clematis
Oh Leadfarmer your garden is gorgeous!
Hazel I have just realised you'd actually commented on my thread. My holly does look very bare and somewhat deformed at the moment, but I trust it will improve in time. One sure thing is that it couldn't carry on the way it was going.
It sounds like you have doubts about both the tree and the climber (ie what kind they are). If it's not clematis montana I guess the treatment - and the problem - will be quite different? And is the tree deciduous? It would be good to see what happens.