It sounds like on the whole, your son is a very bright, good boy. You say yourself that he is getting excellent grades, no complaints from teachers, etc. You don't specifiy what he has been caught 'hiding' only say it was 'things he didn't want you to see'. That could be a LOT of different things- harmless curiosity or something dangerous. Or maybe just something HE was curious about, but thought you might disapprove of.
I am NOT trying to criticize your parenting, but I think it may be useful for you to look at this from another angle. Maybe part of the problem is YOU- not even you specifically, but the fact that you ARE the parents!
5th grade is a pretty normal time for kids to want more privacy and also to want to express their 'private' thoughts in an art journal, etc. Maybe some of these thoughts are embarrassing to your son, or just things he wants to be PRIVATE. That does NOT automatically mean he is trying drugs or other things he should not.
But it is very natural at this age for him to be curious about girls, rock music, etc. and maybe that is something he feels uncomfortable sharing with you. Ask yourself honestly- why do you think that is? Does he assume that you will automatically disapprove or even forbid him from thinking about such things? If so, I think you are doing your son and your family a HUGE disservice.
Remember, curiosity in kids is a GOOD thing- it allows YOU the opportunity to open up a discussion and explain why you think something is ok, or why you don't. Keeping the lines of communication open are critical!!
If you let your child know that you would always like him to come to you with ANY kind of issue or question- even things that might normally be 'taboo' to talk about in your family or church like homosexuality, sex, drugs, whatever- then he is more likely to share his natural questions about such things with you.
If he expects that you will overreact and take things to an extreme- banning his art journal, forbidding him things, refusing to even discuss them- he is MUCH more likely to hide his curiosity from you and to get his information someplace else. Maybe you can suggest he talk to his school guidance counselor or a pastor at church or something like that, if he is not comfortable talking to you about things?
TRUST is a two way street. It sounds like you have always trusted your son to do right in the past and he has not let you down. I believe that you need to give him some more of that trust now. You need to make sure that he knows he can trust you and come to you with his questions without being smashed down.
My answer would be different if you had reported that his grades had suddenly dropped, he had gone through physical changes like not being able to get up for school, had stopped doing homework, had a new crowd of friends you don't know or if teachers were worried about him. But none of those things are happening.
Your son sounds like a good kid. Keep an eye on him, sure. But make sure he knows you still love and trust him to make good decisions and allow him some privacy. It will strengthen your relationship in the future when he is a teen.
Showing him something like this to 'prove' your concern to him sounds a lot like trying to make him feel guilty for even thinking about things. Also, I have to say, I don't think he needs 'Tough Love' just yet. Many of those books and programs are designed for families with kids who are exhibiting really serious behaviors- stealing, drugs, lying to get money or sneak out of the house. I think that to go there at this point with your son would be overreacting. I am just really afraid that attitude will backfire on you!!
Make it BETTER for your son to trust you and confide in you than to lie just to make YOU more comfortable. If you feel awkward or unwilling to talk about things with him- ANYTHING- then go yourself to your pastor or a counselor and get some pointers!! Get comfortable with discussing ANY issue that might come up. Talk to your son and trust in each other! Good luck!