Marriage of Developmentally Impaired Child

We have a son with developmental disabilities that prevent him from having a normal life. He will never be able to live independently. Our rabbi said he is patar mitzvah. He is in his 20s and we have guardianship over him now, and after we are no longer able to care for him, he will probably have to go to a group home. He has good social skills and a job that he does very well and he wants to be as independent as possible. He is jealous he cannot have a fully independent life like most adults. It is obvious he is attracted to women and he wants to get married someday. He understands the concept of love and marriage. We discussed this with both his doctor and our rabbi and they both feel he should not marry. But we feel his pain and believe it would be cruel to deprive him of his desires and we disagree with both of them. We know of other marriages between those who are intellectually impaired that are successful, some who even have children. How should we approach this? And what if we override what our rabbi has told us and take steps to find him a wife?