What exactly do you mean by "run out"?
If he has used up the membership bundled call, text and/or data allowances but the membership month has not yet expired, then he needs to buy an add-on to tide him over until the membership runs its full monthly course, expires and he can start a new membership.
https://community.48.ie/t5/The-Knowledgebase/All-About-Add-ons/ta-p/53111
Or if his membership month has expired then just needs to buy a new membership to continue using services.
https://community.48.ie/t5/The-Knowledgebase/How-do-I-buy-Membership-Add-ons-or-Top-up-on-My48/ta-p/...
You can buy memberships any time but if the current membership is still active and the membership month has not yet expired then any new membership(s) purchased will "stack" to kick in when the current membership has eventually expired.
