Kelly Rowland's son Titan ''keeps asking'' for a sibling.

The 38-year-old singer has four-year-old son Titan with her husband Tim Weatherspoon, and has revealed the youngster is desperate for a little brother, but Kelly isn't sure the time is right to expand their family.

She said: ''Oh my gosh, my son keeps asking me for a sibling. But it's just not [the] right timing.''

And the former Destiny's Child star says Titan is ''very specific'' about his request, as he doesn't want his future sibling to be a girl.

Kelly added: ''He's like, 'We're going to have a brother, Mom.' I said, 'Well, he won't be my brother - he'll be my son - [and] what if it's a girl?' [And he said] 'No, it's not a girl, it's a brother.' ''

But right now, the 'Work' hitmaker is keen on keeping her focus on just one child, whom she describes as a ''very smart kid'' who's ''way smarter than'' her.

She said: ''I'm so happy [about] that because I prayed for it - smarter than me to the point where he'll be speaking Spanish to his friends or someone that might be passing on the street. He's like, 'Hola. ¿Como estas?' I'm like, 'Dude, we don't speak to strangers.'

''He was like, 'I mean, he was just walking by, and I heard him speaking Spanish.' You know, he's that cute. He has a really good personality. Soaks in everything.''

And although Titan might be smarter than her, Kelly is hoping he'll pick up her passion for charity work, which she got from her own late mother Doris.

Speaking to People magazine, she said: ''We're starting that now. I remember I tried to start it at three, but it was too young. I'll break it down for him. This last time I did it, I said, 'Okay, you've got a new, whatever it was, you have to give away one of your toys.' [He said,] 'I don't want to.' I explained everything to him - how there's some kids with no toys, and 'You have three of the same thing.'

''I said, 'If you can't share then I have to strongly think about if I'm going to share what I have with you. Mommy works hard ... and I'll give you the things that you need, like your bare necessities, but I chose to buy you a gift. You have to choose to be kind to someone else.' And so he says, 'They can have it.' ''