1. get a new trainer because that is Bull! Any good trainer will give you help, not say "oh yeah, I noticed you have a problem- let me know how you decide to fix it!!"
2. try the afore mentioned because they are all good tips but also try when you are just riding around the ring lifting straight up on your reins (in a snaffle bit) and literally lifting up your horse's shoulders. When you feel him lift up his shoulders, release your hands back to their normal position and repeat as soon as he drops his shoulders again. This also works for horses who tend to drop their shoulders in a spin.