It wouldn't deplete his contributions - if he is eligible for Jobseeker's Allowance (the means-tested payment), then he would receive a PRSI credit, which would maintain his contribution history for the state pension.
"Stamps used up" refers more to a time limit for claiming a payment which a person qualifies for based on their PRSI contributions. For example, Jobseeker's Benefit, which is based on PRSI contributions, is payable for a maximum of 9 months.
PRSI contributions paid by the self-employed (class S) do not entitle a person to Jobseeker's Benefit, so if he signs on, he will be means-tested.