Don't rub it in. You don't mean to, but that's how it may feel to your older child when he hears you say, "Isn't it amazing how well Jacob dribbles the basketball even though he's so little?" Same with asking your younger child to teach his sibling -- instead, do it yourself, or ask a neutral party, like a camp counselor, to help. You should still tell your younger child you're proud, of course! But also add, "Your brother's working on learning this, and that's good, too."
Encourage him the right way. Tell your child you're sure he'll learn to write in cursive, or whatever it is he's trying to do, but empathize with his struggle: "I know you've been working really hard on this and it's frustrating. But keep it up -- I know you can do it." Share a hard time you had learning something new, or remind him of things he's good at, to boost his confidence.
Look for activities that they both can enjoy. If playtime's getting tense, suggest an art project or a game of duck, duck, goose -- something that isn't so focused on skills.