Having been in stepmom's shoes over and over again I can sympathize.
My Stepson's mom made a point of at least once a year turning into "Suppermommy" for a week and giving him everything she could and promising big presents or trips for the near future. By the time he was 10yrs old he knew all on his own she would probably flake out on him. But, he was still a kid and she could still get him all excited with promises of "this time I'll really do it" "I know I didn't last time, this time will make up for it" etc etc etc etc....
I think, it would be cruel for stepmom and Dad to not let him know what his mom has told them. They don't have to make excuses for her or make up for what she has done/not done. Just let him know exactly what she said and suggest that he talk to her about it.
This kid is 10... not 3... he knows a lot more than most people seem to be giving him credit for.

OH and... Why do so many people think stepmom should keep out of it? They live together. She obviously cares for him and wants what is best for him. Kids (everyone actually) need family... why can't step-kids and step-parents just BE FAMILY and treat each other that way?

There's another issue here that everyone is touching on but they aren't coming right out and saying it.

Never let the kids play one parent off against the other. I.E. stepson asks for a cell phone from Dad ... dad say he has to wait until he is 15, so stepson goes to Mom with the same request. Mom says ok, but later realizes that she can't afford it. If the kid does get his cell phone from either side he will learn that he can keep doing this forever. If mom says no, ask dad, if dad says no, ask mom.

My nephew and niece did that to my sister and her ex. They always got what they wanted from one or the other parent. What they learned from it was that the world owes them a living.

He is now 31, can't hold a job more than 6 months and has 3 kids by 3 different women and a 4th from another woman on the way. And he is always asking one parent or the other (or more often asking his grandmother) to lend him some money. He seems to think that he won't have to pay it back either.

She isn't quite as bad but she can't seem to hold a job either.

The LW should tell her stepson exactly going on. She shouldn't wait until he finds out himself that he won't be getting a phone, and she certainly shouldn't buy him one just because his mother can't or won't.

WOW - sounds like my life... truth be told (as I've gone down this same road with 3 kids) each kid will learn how to deal with it in their own ways. But one thing is consistent in each kid - they learn that mom is full of cr*# almost all of the time. When she does actually follow through on something, it's like fireworks, then fades within seconds, as they've been let down soooooo many times, it's just not as big a deal anymore. Rest assured, what goes around comes around eventually. Problem is, she probably will never even realize it. Some people are just not cut out to be parents. Certainly all of us can create life, but only a fraction can be a parent. The mom who promised the phone is proof of this, whether she can afford it or not. She should put her phone money to an airplane ticket, and spend time with the son she helped create - that will mean more than any phone.

yes, her time is the greatest gift she could give this 10 year old. I agree.