You can only put him as your dependant if you are getting healthcare on the S1 form. To get healthcare on an S1 you need to be receiving uk state pension or receiving an exportable disability benefit.

Entitlement to an S1 does require that you be in receipt of your state pension.

Under the current rules you need a minimum of 10 years worth of contributions and if you say he's just short of that then buying one year could be the solution.

You can buy up to 6 years even after reaching pensionable age but there are some rules to it

I think it’s possible for him to come under the reuniting of family rules, nothing to do with S1 now, and as I’m in the system he’s a beneficiary of mine... I think..
it’s something we’re looking at now x
His time in uk was too long ago to be able to top up. Xx