How to Convince DH to Follow Through with Consequences?

Just read your Edit:
"Mnemonics" is a method of remembering and memorization. You could try that. It is widely known and you should find info about it online.
I actually did a study on it in college.

If his "problem" is that he is forgetful... then I really don't think he is on-purpose trying to disrespect the 'rules.' He can't remember... to begin with what was asked of him.
So maybe then, punishment will not work. It will not suddenly make his memory good.
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If there are no rules from Dad... your DSS will not be learning any responsibility nor respect.
And, your Husband has to be a "Dad" ... not a "friend." He is a parent.
That is his role.
Bottom line.

If he is 'afraid' to instill any kind of rules/consequences and make excuses for his son... then he is doing him a dis-service, and he will then grow older and his son will not have learned anything.... about just plain respect and 'responsibility' in a family and the real world.
How will he be when he has to get a job? For example.

He is 15. And if he wants more freedom/choices/privileges ... then he has to prove he can act accordingly. If he thinks he is a bog-boy... then prove it. And he is not the one deciding what/when/if he does things... he is a KID. Not a grown up.

Does he do chores in the house? Well he should.

Or maybe your DSS is just forgetful??? Then work on that... or YOU call him intermittently, to check up on his status and "remind" him.... of things.

Is he overall a good kid? Or just forgetful? Or is he a bad kid? How are his friends?
Communicate with him, but yet you don't want him to start keeping 'secrets' about himself or his whereabouts or activities.

Keep consequences in line with the wrong deed.

all the best,
Susan

Oh boy. If I let me girls get away with this tyope of thing, they would ALWAYS say they told me. I try to be fair and maybe, if there is a slight possibility, shorten the grounding to a day or missing out on an activity. One time my girls were supposed to go to a church for some activity they had. The church is 2 blocks away so no problem with it. My girls were 10 1/2. First, they tell me they will go meet the other girl at their friend's house. Then, they say they are waiting for her on our steps. Instead, the girl that was going with them comes by my house over an hour later looking for them. OMG. WHAT! anyway, I went looking as best I could since my newborn was asleep. The other friends' mom said she would check for them. It turned out they ha gone by this one firends' house as they said after they told me they were going to wait on the steps. Then after the girl came looking for them, she fouind them at another kids house and they went over to the church. This is all almost 2 hours after they were supposed to. The one friends' mom found them and made them come home. They were embarassed to be pulled out of the group. But they got grounded for 2 days. It would have been longer but I did kind of know that they were supposed to go to the church. Otherwise it would have been a week. They know after this has happened a handful of times that they MUST tell me when plans change or they go from one friends house to another.

Consistency is the key! You've clarified the rules and even wrote them down. Kids have to accept the consequences. I went through this with my son (just turned 15). I had to repeat most expectations ( and still do)several times to my son because his working memory is terrible, but I learned that once he knew that there would be consequences and I wouldn't accept any negoitiations, he tried much harder to recall the rules... stick to your guns.

this doesn't exactly answer your question, but i think you need to write down what your son says to you about his plans, then repeat it back to him. then there can be no dispute over what anyone remembers. if he breaks the rule and tells you he forgot, you show him the paper. he still pays the consequences. hopefully it will motivate him to do something himself to remember. 15 yr olds are capable of writing themselves notes, putting reminders into their phones, setting alarms on their watches, etc. get him involved in solving the problem.

I'm responding to your follow up. If DSS has trouble remembering, maybe you need to address this. I don't have any suggestions on how to do this other than when he does call, ask him questions like, "will you still be home at 5:00?" or in the morning say "we expect you home right after school, don't forget to call if any thing changes".

As fas as DH is concerned, I be very annoyed if I told hubby DSS did not say, and DSS said I told her. Hubby needs to be on the same page with you, or your disapline efforts will go down the drain. Wish I had better advise. Good luck.