None of the problems the teacher used as examples of an issue would strike me as worrying for a child of 6.
He may well have an issue in math but it's likely that is related to needing a different style of explanation. If he is art focused, the instruction to colour parts of a chart may have come across to him as a colouring activity instead of a primarily math activity.
His reading is perfectly normal, and a page of writing without punctuation is also completely normal. If the content made sense and he is not showing signs of misspelling basic words like 'with' as 'wiff' or 'wiss' he's probably doing fine with his short term memory too, and absorbing some of what hes reading. Spelling 'with' as wiff or wiss would indicate he's still sounding out words he should be able to retrieve from short term memory.
The beautiful handwriting is associated with his focus on art. He is clearly a child with strong hand-eye coordination and excellent fine motor skills.
Children develop different aspects of reading, writing, and comprehension skills at different speeds. It can be an unbalanced progress. Sometimes decoding proceeds well ahead of comprehension or it takes so much focus that comprehension requires a second or third read through. Sometimes punctuation falls by the wayside when other parts of the skillset are coming along well. None of that is an indication of an overarching problem.
Please remember that children in other English speaking countries are not being academically hot housed the way British children are in early years education. The teacher's expectations are unrealistic, imo.
I'd take it all with a big grain of salt, in other words.
Keep him interested in math at home. There are math aids that can help children visualize concepts (cuisenaire rods, for example), and plenty of activities available online too. Baking is loaded with math skills - concepts of half, a third, a whole unit, relating volume to numbers, adding, subtracting, dividing, doubling, etc.