5x5 isn't particularly high volume, Sheiko most definitely is. Your issue is more your nutrition than your training, eat enough and you'll grow and get stronger no matter what you're doing. Don't eat enough and Sheiko will absolutely bury you. Seriously, eat more. "I'm young" is a bullshit excuse.
If anything it means you can convince your parents you're growing and they'll think it's normal to provide you with a ridiculous amount of food free of cost to you. Adding size when you're an established adult often means cutting back elsewhere in the budget - I know my food bill close to doubles when I'm looking to add more size. Let me reiterate, eat more.