Normally the kidneys filter back glucose into the blood stream . When the concentration of glucose gets too high it can't do that so excess is spilled over into the urine. The level in most adults that this happens is at around 180mg/dl (10mmol/l) but can be lower in children. It's telling you what you already know that his glucose level has been high .You are doing the right thing to lower the levels. The process is dehydrating which is what causes the thirst. (he may still show some glucose in his urine after his blood glucose has gone down because the urine was 'made earlier'. )
edit: obviously keep a close eye on his levels today and if they stay elevated recheck for ketones/contact your nurse/doc for advice . The rise may be a one off, maybe something he ate earlier caused a later rise, it maybe that he is developing a cold or some sort of 'bug' or it maybe that his insulin needs are changing.