i think it would be wee - i find that they can wet themselves in such strange positions - up the front, up the side, up the back - once i found DS with a huge wet patch under his arm... and it was wee...
i would thik it is unliely it is sweat - if it was i would think he would be sweating elsewhere (neck, torso would be hot) and he would be hot... if it is sweat - and this much - i woud get him checked as i would not think this is normal - but the wee certainly is!!!
how hot is the room and how many layers is he wearing?
perhaps the nappy does not fit properly - i found huggies too big/wrong shape to start with so i used baby love for a few weeks and this helped with leakage. i still find (at nearly 10 months) that at diff times DS fits diff brands better and we get less leakage w diff brands - more shape i think than weight.
congrats and enjoy!