Custom JavaScript verification is the way to go:
var resp = SSI_GetValue('[% QuestionName() %]_3_other');
var regex = /[0-9]+/;
if (resp && !regex.test(resp)) {
strErrorMessage = 'Error.';
}
In line 1, replace the "3" with the item number of the other specify. Line 2 may need to be replaced depending on exactly what inputs you want to accept:
var regex = /[0-9]+/; // any number of numeric characters
var regex = /-?[0-9]+/; // an optional negative sign, then any number of numeric characters
var regex = /([0-9]+.?[0-9]*)|([0-9]*.?[0-9]+)/; // one or more numeric characters with an optional decimal point
Hopefully one of those regular expressions is acceptable for you. If you provide exact requirements of what inputs are okay, I can create the necessary regex for you.