Dh left school at 15 with no qualifications and I was distinctly average in school but my children have all done very well for themselves in the end.
The highest earner of my three adult dc had only just started speaking at school at 6, never mind reading or writing but he got better Higher/Advanced Higher results than his older brother who was considered a “high achiever” in primary school. Then their much younger brother, who had the Covid years at the start of secondary school and was diagnosed with ASD and DCD finished school with a, fairly unexpected, near perfect, set of exam results. It’s far, far too early to tell who they are going to be at 6. Mine were also all rubbish at sport except swimming at which they did well but doesn’t count for anything at school!
My advice (which you probably don’t want but I will give anyway) is firstly, stop worrying and change your mindset, accept that he’s running slightly below average at the moment and look at whether there could be anything affecting his learning. Make sure he has had his eyes tested by an optrometrist and get his hearing checked. Does he have difficulty writing because his hand get sore. He could be hyper-mobile or just lack muscle strength. Then lots and lots of learning through play. Baking, counting with toys or object, cut and stick to match sounds and words that he is doing. Practice writing them in the sand, flour whilst baking, bubble bath in the bath, chalk on the drive etc. Read to him and with him all the time, signs when out, sauce bottles on the table, comics etc. consider music lessons once he can sit still long enough. Ds1 and 2 started with violin, Ds3 with piano, he’s a great pianist now and loves playing for pleasure. It’s another string to their bow (pardon the pun) and definitely helps academically. Most of all, boost his confidence. Tell him how clever he is and praise every little achievement. He will be fine and so will you.