Your trainer is probably just concerned because these big horses can be more prone to lameness problems. Lunging for too long or on tight circles is tough on any horse's legs, and it's worse for these big horses. But if you lunge on large circles and don't do it very often, he should be fine. The key is not to overdo it.
And the reason lungeing can be hard on the horse's legs while riding in circles is not considered to be is the repetitive motion. I often see people lunging a horse round and round to the left for 10 minutes, then they switch and do 10 minutes of lungeing to the right. You would never ride this way, or at least I hope not! When riding, you might ride a few circles, then go down the long side, then across the diagonal, circle again the other direction, etc. It's the round and round on the circle that stresses joints when lungeing- the repetitive motion. The horse's inside hind leg carries a lot more weight, and he experiences torque on his legs, especially if he's unbalanced on the lunge. When you lunge, it helps if you switch directions frequently and also alternate circles with taking him down the long sides (you'll have to run to do this, but it's good exercise for you!).