Nearly 5 year old still holding pencil in fist

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Ds has just started reception after 2 years of pretty much full time nursery. I know his nursery teacher mentioned it once and has tried to help but he is still doing it.
He hasn't even tried the three finger stage but holds it in a full fist.
Should I be concerned about it?
I can't see any other issues. But is there anything else I could look out for?
He was fairly late in some milestones (couldn't sit alone by 9/10 months). He has two sisters who have lovely handwriting and learnt pretty early on.

Focus on some activities for strengthening his hands like moulding play dough or squeezing out sponges during water play.
Also fine motor skills like peeling off small stickers and threading beads on a string which encourage a tripod : pincer grip.
Using sprung pegs in pretend play/ games is good too. Also easy sticks ( joined chop sticks ) are fun!
Get Some foam or soft rubber pencil grips which slot on to pens/ pencils to encourage the correct grip and ensure feet are supported/ flat on ground when sitting.

Take some steps to correct this now with help from school to ensure your son’s learning can stay on track.

From my daughter's occupational therapy sessions... writing on walls or vertical chalkboards - the vertical angle of the wall means you can't really write without your hand being guided into a more appropriate grip.

My daughter did this too, it was corrected by year 1. Your right to do some fine motor work with her that will help . Also physical play to strengthen muscles x

Thanks all. Will see what teachers say also at Parents Evening.