well, to quote one of my favoure movies 'time has a way of changing things' so a binding spell today might seem great, but you should love him for him, and if he's immature now and it's bugging you, in 5 years you might despise him for it. not that i'm saying in that time he won't mature.
to strengthen love you don't really need a spell, it wouldn't hurt, but i think it would be more beneficial if you two just spent time together, no distractions, just be together. granted even if you cast a spell, you'd still need to put some effort into the relationship to aid it. if you mean faithfulness, i know of a spell or two [not giving out though because it's sex magic] they're binding spells, and i don't think binding spells are good in the long run really... unless it's some life or death thing, and even then.
if i were you, maybe cast a simple candle spell, get a red or pink candle, carve both your names in it and circle it with a heart and burn it. watch the candle and visualize the two of you together, happy [maybe with a pink glow around you] then you need to work on your relationship. take him out to a movie, go for a bike ride, a walk in the woods or something you both enjoy just the two of you. also don't know your age, but just remember, time has a way of changing things, in a few years your feelings might change, he might change, you might change, so don't do anything crazy just yet... unless you've been together 10 years and he's 29 going on 13 [if you understand what i mean lol]