Practice using a lot of grid work.
And make sure he understands half hault. Make sure you are not in front or leaning forward over the second fence so he can lift up through the shoulders.
Make sure you sit inbetween and sit tall asking him to wait and shorten his stride.
A placing pole might help between the fences when practicing at home. It limits the area where the horse can take off so he's more likely to get it right
I have an 8yo gelding who had the same problem, and still does sometimes! What helped him was that I didn’t jumped bigger than 60/70 (he’s got a big jump) and always waited for him to chip in a small stride at the fence! Not fun but it worked🤷🏼♀️ A lot of pole work too helped a bunch! Hope this helps somehow!