Half fat should be fine - I believe there is more protein in full fat but don't think that it's really an issue - he can have milk or he can't, kwim?
I saw on your other thread you said he's been a bit of stinker today and also has not had a BM yet - both of those things could be reaction related or they both could just be normal things that are totally not related, things that you wouldn't ever chalk up to anything at all if it wasn't for the milk, right? So as far as giving him more tomorrow or not, I would say either tomorrow or the next day - if you think that today was potentially a reaction, then I would not do tomorrow, see how he is, and then go for another 5ml on Sunday. See what you get. Then unless reaction is clear cut, go for another 5-10ml on Monday. (either 2 separate 5mls or 10mls at once) If he's still fine then, I would proceed slowly but confidently!