First, I think it is good that you asked the question.  You may not like what you hear; however, that is part of the process.  You need to sort thru the answers you get, and decide how you will proceed.  remember, any advice on this board is worth exactly what you paid for it....

I really think that bushton gave you some good advice - there is value in having your son step away from your operation for a couple of years - I would say 2 years minimum, but that is negotiable.  I have seen others do this, and I believe it is valuable to have experience beyond your operation.  I don't know that I would require additional schooling - it is good; but working for someone else can be just as valuable (depending on whom and where). 

To answer your question, if you are intent on bringing your son into your operation, I would encourage you to think about bringin him on as a paid employee - set a time frame to it, say 2 years.  IF all works out between the 2 of you in those 2 years, then think about a partnership, or whatever.  The value of this is that if your son decides to change his mind,  then you are not locked into a more concrete business structure which you will need to dissolve. If you bring him in as an employee, you can pay him a rental/lease for the equipment he owns that the operation uses.  Of course, he could be an employee working for you, and still rent additional land, which he would farm on his own.  If you pursue this kind of detail, you would need to work out details of how he would rent equipment you own, and your farm would rent equipment he owns, and how all that will come out at the end of the year.  (Maybe this doesn't make sense???)

Anyway,  as I mentioned above, all the advice is worth what you paid for it. 

My last comment is that if you have a son who you bring into a partnership at 18 years of age, and it goes smoothly, and he stays for 40+ years, then you have a very special son - and you might just have.  It's just there is as much chance of problems with what you want to do, as it is to go well.  Just think long and hard about what you want to do.  

Have a good day.