Hello...My internet is up and running much better now. I see that no other expert has assisted you so I am going to step in.
When you tell your son to do something, use simple sentences of one syllable words of only a few words like: Do not climb. Then have your son repeat what you have said. If he stops what you told him to do, great. If if continues to perform the action then say, "Mommy said "Stop. Do not climb because it is not safe. ' " Then remove him from the situation. When you are saying these things, use an authoritative voice even if he starts laughing. The main goal when you discipline is to teach him rather than to punish him. If he climbs anyway and starts laughing, take him and sit him down in a chair with you facing him in a chair for 2-3 minutes. Tell him that he is being disobedient and that you do not like when he does not follow the rules. Ask him does he think that he can follow the rules. Then have him show you how he can follow the rule(rules).
Whenever you "catch" him behaving really well, give him a sticker on a chart and REALLY make a big deal about what a great job he is doing. Then after 5-10 stickers, reward him with a trip to the park, ice cream, frozen yogurt, or whatever he really likes. You are going to have to be consistent with this or else it will not work. The main key is that positive behavior must be focused on much more now. Your son knows how to push your buttons and therefore he laughs because he thinks it is funny. Stop giving all of the negative behavior the attention and watch your son be that wonderful angel that he once was.