The work isn't actually harder. Help your son organize his study habits to cope with the new challenges. Help him figure out how long each thing will take, and which tasks he needs to do first. Don't sit there and monitor him constantly, don't do his work for him, and don't lecture him. Let him make his schoolwork comfortable for him.
Different children study in different ways.
Some need regular short breaks, others work for a long time and don't like interruptions. Allow your son to study in the way which works best for him. But don't let him stay up past his bedtime doing schoolwork. Make sure he starts early enough to get his work done and have some relaxation time. If there's too much work for the time available, that's the teacher's problem.