Neha,
You can also order the chest strap that will fit him, and then just put the heart monitor chest piece under the chest strap on your bra. This works great, for me, as it's hard to find an x-small chest strap for myself. So I generally just put it under the chest strap of my bra and it stays there until I take the bra off. No matter what activity I am doing, I've never had it fall out of place. And the readings have been just as accurate under the bra, as they are with a strap.
HTH,
Kit