Well, then I guess the problem resides in the severity of the complaint.
When he says it is difficult does he merely mean he has to work a bit in order to learn the material? If that is the case then there isn't much you can do about it. Everybody needs to learn things eventually, and learning does take some effort, at least more so then playing with legos. The complaint about school being boring may also be of minor concern, as in he just gets a bit bored during story time. If this is the case then I would take time to remind him of all the benefits of going to school. I.E. learning how to read helps him to read things which interest him such as game guides to the Mario games. Also, school lets him spend time with his friends or other enjoyable activities (assuming of course that he finds at least something enjoyable at school). It may also be a good time to talk about making good commitments to do the right thing even if it is a bit hard. For example, dad goes to work every day, so should your son.
Conversely, if by too hard he means that he is completely confused, bewildered, lost and getting frustrated then you have a more serious problem. Likewise, by boring he might mean something more then just a little boredom. He might mean that going to school is a horrible, soul crushing experience that is draining his will to live. Trust me, I've been there, done that, not fun. If that is the case you probably need to take what he is saying a bit more seriously and find out what is causing the problems.
So, I guess I would first try talking to him and finding out specifically what he means by 'too difficult' and 'boring'.