I had a very similar problem with my DS (turned 5 in July) and a similar reaction from school. I can't believe your OT hasn't told you about pencil grips! You can buy grips to help with the tripod grasp - even a very general store like the Early Learning Centre do a pack of 3 - and you can also buy triangular shaped pencils. As others have said there are a raft of different shaped grips, slopes etc that the OT should be able to advise on.
It might also be worth your doing some 'finger disociation' exercises with him - this basically refers to the ability to move one finger independently of the other, starting with the index finger, to help strengthen the fingers. So for example games with finger puppets, use one finger to operate the trigger on a water pistol or similar, have races with small objects like marbles pushing a marble along with one finger. Another good game to strengthen the pincer grip (which is essential to help them develop the tripod grasp on a pencil) is a simple game using clothes pegs i.e. put say 10 pegs into a bowl, get DS to lift them out one by one and clip them to the edge of the bowl using thumb and index finger, then take them off and replace in the bowl.
Once the grip has strengthened and he is holding the pencil correctly (our OT taught DS to 'make a snappy crocodile' with the thumb and index finger, tapping them together before picking up the pencil with them, to try and encourage him to hold it correctly) then try lots of work to encourage copying straight lines, zig zag lines, wavy lines moving from left to right across page.
You don't mention how keen or otherwise your DS is to try mark-making and writing work - half the battle on this I think, particularly with boys, is getting their interest. Lots of boys of this age are simply not interested! The OT that my DS did used to encourage him by, for example, printing worksheets with cars and aeroplanes etc on with dotted lines in either straight, wavy or zig zap lines - i.e. follow the race track or help the aeroplane fly across the paper. Colouring in is also a good one as it helps develop control in trying to stay between the lines. Problem being of course that a lot of boys hate colouring in, but again if you try and pander to their interests and find pictures of their favourite cartoon characters or whatever it might work.
Failing that, my best tip is bribery - i.e. a sticker for every session (no matter how short) and then perhaps a small treat once he's got 7 stickers. Alternatively an immediate food related reward always works well for my DS - biscuits or Haribos usually - though obviously this can backfire if your child reacts adversely to sugar - luckily DS doesn't!
If it is any help, at 4.5 my DS was really struggling with all of this. Used to clutch at the pencil like an 18 month old, wrapping all his fingers round it and just scribbling aimlessly. Now (less than three months after completing a short course of OT last term, and with lots of practise over the summer) he is gripping the pencil correctly, colouring in much more accurately, can write not just his name but form almost every letter in the alphabet from memory, and writing not just whole words but complete sentences. Okay, his writing's not the neatest but we are getting there, and when I compare where he is now to where he was then I can hardly believe the progress.
So don't let school panic you too much. 4.5 is still very young, and he's a boy (even very able boys generally develop fine motor skills at a slower/later rate than girls), and there is so much you can do to help him. Hope this helps - sorry it's so long but this is a subject close to my heart!