sick Lavender orp, skinny and doesn't move much.

[Wanex]

I need help, my 3 month old lavender orpington roo seems to be sick, he looks normal, but I think he should weight more, he also gets tired of standing up, and he made some weird noise and he moved weird right now, im worried because he is next to my buff orps and im worried he might get them sick as well, any ideas of what it could be? Thanks.