Either put him up as available for loan and only lend him to teams where he'll be "first team", or just play him in bankers like cup games against lower league opposition. Or even bring him on alongside Taylor for the last 15-20 mins of a 3-0 up win (just remember to play defensively).
Other things to remember are: check his language skills; he's more likely to play well if he has a basic understanding of English. Plus his favoured personnel can be a good indicator of bedding in. And of course his training performances.
Was he match fit? I find my players making silly errors when not match fit.
Can't really add much more than the others, although I would suggest that younger players are often less able to move to new teams and settle in immediately. That's because they usually haven't developed the mental characteristics to adapt to playing in a new country or team.
I have usually found that the best way to introduce a younger play is to sub him in during most games for the last 30 mins or so, especially in games where you are already winning comfortably, to allow him to bed in. Then after 15 or so sub appearances, if he has good morale and match fitness, I would start him at home against a lesser side who I expect to beat. I'd see how we went from there.