Yeah, you need about a week off school, until the spots are crusted over. It is very boring (well was for us!). Luckily they are not too poorly, but you can't really go out so it's rather dull....
Should be fine for the birthday party, unless he has siblings who will probably get it in about 2 weeks time, just in time for the party.... there's about a 14 day innoculation period.
He will probably be ok for his grading, providing the spots are crusted over, but his spots will still be very visible and you may need to reassure others that he is no longer contagious.
I didn't take mine to the docs- it's SO contagious I think they don't normally want you there! - however if you have any concerns call your GP for advice.
My son (4.5) had it really mildly and didn't really need the calamine or piriton or anything. My daughter (2.5) had the more typical version, loads of spots, really itchy. We coated her in calamine and gave her piriton -or tescos equivalent which really helped with the itching.
Good luck with your week of quarantine! At least it gets it out of the way...