Speaking to Heat magazine, Cox said that the 18-year-old - who was recently photographed sharing a kiss with Emma Ostilly - does not want to "tie himself down".
"I know I'm biased, but he's genuinely a lovely lad, just a normal 18-year-old doing abnormal things at the moment."
Describing him as a "very romantic guy", Cox continued: "He'll make a nice boyfriend when he's ready. He's very sensitive and isn't a Jack the Lad. At the moment he doesn't want to tie himself down.
"He had one or two girlfriends when he was younger, but no-one very special. I'd love Harry to have a family one day, and he'd love one, too. He'd make a fantastic dad."
On his previous romance with Xtra Factor presenter Caroline Flack, which made headlines over the large age difference between them, Cox said that she had not interfered: "I have met Caroline Flack, but I don't want to go into things about the two of them.
"That's over now. I never really thought about the age gap, but I had an age gap with Harry's dad (she's 44, he's 54) so I suppose that's what he saw. I don't think it's very important, to be honest."
Adding that being in One Direction is "the best thing that could have happened to him", Cox concluded: "He's a mummy's boy. Sometimes, he phones up to five times a day. When there's a time difference, he tends to text saying, 'I love you, Mum', or 'I miss you'. He's still my little boy."
Read the full interview in the new issue of Heat magazine, out now.
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