How will i teel him that i donot love him?

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i met a guy in the net, and he sent me his diary.And he told me about his mental incapacity and he lacks self-confidence even though he is an engineer i feel sorry after reading his diary and what is on my mind is to give him moral and spiritual support.And when i'd done that, i have noticed the big change in him.After 3 months of chatting, he proposed for marriage, cos of my willingness for him not to be disappointed i accepted the proposal to the fact that i dont have any feelings for him.And in our every conversation he always reminds me of the proposal.I think of telling him the truth that i donot love him all i want is friendship, but i always change my mind cos i dont want him to be depress and i dont want him to live in a miserable life again.I made this cos i have a sister that is very much like him(mental incapacity).I made an experiment on him so that i will know how to treat my own sister.While we are talking i have noticed that he is well now, you will not going to think that he has a mental capacity cos he talks so smart, he is very intelligent and he gains self-confidence already and i am thanking God cos He have used me to help this person to lift up his self and face tomorrow with a greater hopes.My biggest problem is that he keeps on telling me that he loves me.How will i tell him that i donot love him without hurting him so bad?