Yes, it's a heck of a good deal if his Retirement Account can have a bigger balance. (It's his Retirement Account specifically. Once you reach age 55 there are no more directed Special Account top ups.) Presumably he's nearing retirement, or at least would like to retire if he could, and he's no longer a 30-something worker with decades of earning, saving, and prudent investing ahead of him to build a retirement nest egg. I really can't think of anything better than CPF Retirement Account top ups for older (ahem, mature and experienced) workers who haven't formed much (or any) independent retirement nest egg yet. (In some classic cultures and contexts, children and grandchildren -- and claims on their earnings -- are the retirement plan. That approach has always had its limitations and works even less well in modern Singapore. I'm not a big fan of it.) You just can't beat the RA's combination of 4+% interest and safety for somebody in that position.
So yes, he and everybody who loves him who can help out ought to be slamming lots of dollars into his Retirement Account, starting with at least maxing out available tax reliefs which are available as long as his RA is below the Full Retirement Sum. Here's an example assuming all these individuals pay some income tax:
1. You top up his Retirement Account by $7,000 for tax relief.
2. Your older sibling does the same thing, also for tax relief.
3. Your younger sibling is cash strapped at the moment but still qualifies for tax relief, so you hand your younger sibling $7,000 to top up your father's RA. Your younger sibling then pays you back a couple months from now, maybe sharing some of his/her tax relief with you.
4. Your father tops up his Retirement Account by $7,000 for tax relief.
Everybody does this on the second to last business day of this month (January, 2020) using PayNow QR, I'd recommend. That'll mean that the top ups start earning RA interest from February 1, 2020, while also earning whatever bank interest the funds are currently earning for most of January.
There are also possible CPF transfers available. For example, if your father has funds in his OA and SA, he can transfer those funds (drawing first from his SA, then his OA) to his Retirement Account. The portion drawn from his OA will then be upgraded from 2.5% OA interest to 4% RA interest. You, your siblings, and any other qualified family members may also be able to transfer some of your CPF dollars to your father's RA. See
here for details on those rules. Those of you who have used OA dollars for housing might consider paying some or lots of those OA dollars back if the transfer to your father's RA would draw from OA, then do the transfer.
It's best to do any such transfers after cash top ups for tax relief, but transfers should also preferably be done within this month (January, 2020) to start earning RA interest as early as possible.
What does a fatter Retirement Account buy your father? It buys him a more generous income stream starting as early as his 65th birthday month, although he can defer the start of that income stream to as late as age 70. If he was born in 1957 (which is probably correct based on his current age), he'll be able to choose among four payout variations (unless he's somehow already made a payout decision): the classic Retirement Sum Scheme's fixed term payouts (which end at age 90 -- Happy 90th Birthday?) and the three CPF LIFE payout plans (with monthly payouts that end only when he ends, i.e. that last for the rest of his life). That payout plan decision comes later, a couple months before his 65th birthday at the earliest, and is a topic for another day.