Well done for wanting to take him on! Just a wee hint to start with though, they are not 'Aspergers' teens, they are teens with Asperger's Syndrome. I'll assume that was a typo! 
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My DS (now 16) needed to find a work placement in year 10. His school normally arranges placements for pupils with employers who are H&S and CRB checked etc, but after my very bright and polite son was rejected by 3 different employers, we realised that his Asperger's diagnosis meant no-one was willing to offer him a placement. 
The school, to my absolute disgust, washed their hands of the situation and said he would have to come into school each day and do extra work instead. 
Well, I wasn't having any of that! I'm a mature student and I volunteer at a local museum. One of my university lecturers is also a curator at the museum, and as DS is a history geek like his mum, she kindly offered to give him a few days work experience. I ran through the following points with her:
DS is a charming boy but is a bit of a space cadet has a lot of trouble understanding retaining verbal instructions. Written instructions are best for him, he can refer back to them and ask for clarification. People with Aspergers can be very literal in their interpretation or instructions. Clear unambigious instructions are best and ask them to repeat them back to you to make sure they know what they are doing.
They can also be very anxious about making mistakes, and become disproportionally upset if they realise they have got something wrong. Pointing out errors needs to be done tactfully, along the lines of: 'perhaps you could do x instead of y, because of abc'. Aspies often like to know why things need to be done in a particular way because they like to understand how things work - this includes systems and processes.
Aspies often work best away from sensory distractions. A noisy working environment, or one that is too hot or cold can make them very stressed. Some Aspies develop tics and twitches when nervous - just to ignore them.
Don't expect them to make much eye contact or initiate conversations. Customer-facing roles may be difficult for them. A task which requires prolonged intense focus and precision can often work well - many Aspies work in IT for this reason. They can often surprise you with interesting solutions to problems - sometimes it's useful to have a non-neurotypical perspective! 
My DS spent most of his time at the museum working unsupervised, writing out index cards and record-keeping - jobs he admitted he found dull, but very easy to perform well. He has an excellent telephone manner because I have trained him to write out a 'script' before making a business call, listing all the points he needs to raise and noting down responses.
I would advise asking your young placementee if there is any 'reasonable adjustment' you can arrange to make his working environment easier, such as a desk somewhere quiet, facing away from other workers, or regular feedback on his progress.
Good luck!