Please watch out for Pedja Krstin in main draw, who I have started working with. I have got him an NCL team in the UK and am now looking for teams in Germany, Italy and France, so CD if you know of any French teams that may be interested, please let me know.
Otto, given his age, ranking and the fact he's Serbian (approved of!), I wouldn't have thought you'd have any difficulty at all.
But, unless they've changed the rules, it will be for next year (you can't add new players after end-September (maybe end-October - can't remember), could check if you need).
So you have to sign with the club by Oct. 2016 to play the 2016-2017 season.
Often it's good to choose a club that has Serbian players already (it means the club has a connection of sorts, and it can be easier).
I'm afraid I don't know any clubs out touting for players - there's no official website or anything - but emailing the presidents of clubs in a geographical area that is convenient, playing for a league that interests him - does he want to play number 1 or number 4 or doesn't care? - would be perfectly acceptable, I think. (Do the email in French).