Hi,
you can do this with javascript

jQuery(document).ready(function(){

    jQuery('#member-registration .btn-primary').attr('disabled','disabled');

    jQuery('#jform_ALIAS0').change(function(){

        if(jQuery('#jform_ALIAS0').is(':checked')) jQuery('#member-registration .btn-primary').removeAttr('disabled');

        else jQuery('#member-registration .btn-primary').attr('disabled','disabled');

    });

});

replace ALIAS with alias of field

In which module i must put this code?
I tried with name.js module but doesn't work fine: the submit button is always disabled.

You can put this code in any javascript file, name.js is ok.

Have you replace the correct ALIAS of field?
if alias of field is checkbox_tos you will replace from my code this:

jform_ALIAS0

with this:

jform_checkbox_tos0

Ok. Work fine.

Will I lose those changes when you will release a new version and I will update the system?

Yes, I recommend to put this code in template JS file