Visit

Should be fine, if he was deported only due to over staying (later must have fined for his overstaying). He can visit without much worried (carrying his visa and other necessary documents).

Thanks Surya for the prompt reply

He was explained me (he was got caught due to theft in a shopping mall and arrestedby police after judge order huge amount or 14 days and then deported ) but not fined anything.

now he is worried about to fly with his family too for a genuine reason.

So, that means he was deported not because of overstaying but due to major theft. But here I can suggest you should contact ICA to check if that case is still open (most likely no, as after 6 years all minor cases closed but always better to check with ICA and get confirmation). Once ICA confirmed then he should plan to visit Singapore (not before that).

Do you know any site to do this and check further verification

Check the ICA webpage for their contact details and call or write to them.
Or contact your nearest Singapore embassy.