Thanks again for all the positive replies. I know (most of you anyway I assume) aren't medical professionals, but it's reassuring to get support from other parents of toddlers.
The jump in percentiles is the part that worries me too. His height does the same thing - he'll be 25th percentile, have a growth spurt and be around 75th, then no growth for a while and fall back to the 25th again.
When I look at his body, he's just like me as a kid. Small shoulders, slim arms, pot belly, and big bum and thighs. I know a toddler can't be muscly, but his bottom and legs are very well shaped and firm. That is the most solid part of him. His dad and I enjoy weight lifting, and he loves to copy everything dad does, so we let him lift little kettlebells, and he does squats (we don't make him do anything, he just wants to be like his dad).
I just find it so ironic that I've moved to where my MIL lives, and every time she sees Thomas she's so concerned about how "tiny and skinny" he is, she harps on and on about it! She thinks because he's not in size 2 clothing that there is something terribly wrong with him. Every time I see her she pulls a sad face and sighs about how small he is.
His dad has a BMI above normal even though he's got very low body fat, he's very muscular with a flat stomach and abs, so I don't take BMI as being something to really worry about...well except mine as I know I have a lot of weight to lose.
It's funny, as a breastfeeder I spent his first year worrying he wasn't heavy enough, now I'm worried he's too heavy, ugh.