Hi
The normal adult weight range of guinea pigs is about 800-1800g - that is a lot wider than most owners expect because they focus on the narrow average line of 1000-1200g in the middle of the road and no on the width of the road.
As long as your boy is a healthy weight/size, then there is nothing wrong with him. My Pioden was bordering underweight according to his own BMI at 1500g and could have easily carried 1700g for his ideal weight. He was my biggest piggy and covered my whole underarm and hand while my smallest, cousins Llinos and Lleian, are just touching 600g from below.
Pioden was just picky at his eating as well but as he lived to around 8 years or so, it quite frankly didn't matter in terms of a good life and extended life span.
More of my longest lived piggies come from the kerbside of the weight road and not from the dividing 'average' line in the middle of the road when I look back on my coming up to 100 piggies going through my life since my childhood.
Please try not to focus on numbers, expected behaviours and potential problems - they are in your own mind. As long as your boy is healthy and happy in himself, then there is nothing wrong with him. Try to cherish him as he is with all his unique quirks because he is perfectly OK and just himself; but he is perfectly fine in being himself and not a poster advertisement model. What makes guinea pigs so adorable is that they are all unique and different; and that as far as guinea pigs go, rules are there to be broken.