I'm most sorry for you that your husband isn't on the same page with you. It sounds as if he needs a strong but gentle touch from the Lord. Just a thought from what little info you were able to provide, but: might some of his problems stem from his mother's attitude?
I think you need to STOP the business of the older ones not making good on their promises to the little one! Take away their video games IMMEDIATELY if they use them to tantalize and torture him (and then go on to use it to satiate their OWN selfish 'lust' for entertainment)! YOU need to 'make the calls' on that one.
I'd also caution you that if you spend much energy trying to get the older ones to 'like' the younger one, it'll probably backfire and they will accuse YOU of 'playing favorites'. It's hard to love our kids all equally (and virtually impossible to 'like' them equally! There are simply some who are more 'compatible' with our own personalities than others! We have 4--all VERY different, so I know.) Each kid requires totally different KINDS/TYPES of parenting, and that's why it takes TWO involved parents to make a family work properly.
Grandparents can be the most wonderful thing in the world, but unforunately, the experience is only as good as the person. When I was born, I only had one grandfather living, and he was EIGHTY! He died when I was 6, so I was really thankful that our kids had all their grandparents for at least a few years. Now WE are grandparents! LOVE IT! But what a responsibility . . .
Another thought: Maybe 'Grandma' simply gets along better with teenagers than with little kids. That's a real possibility, so if that's the case, when the younger one gets older he can look forward to having the same kind of relationship with her.
I'm proud that you're at least doing something to 'equalize' and alleviate the situation by going along when the older boys go. I'm afraid I haven't been much help, but my thoughts and prayers are with/for you!