If there's a decent, guitar-oriented music shop nearby (t'would be useful to know what region that may be...), how about asking them for a 'gift card' or similar, and arrange for your brother to come along and choose one, up to the value you've set with the shop..? T'would be nice if they knew, beforehand, that he's 'lefty', so as to have something to propose.
Another big factor would be his tastes in music, and experience level. Does he already play guitar..? Would an acoustic be suitable, or an electric (which implies having an amp...), or even (sacrilege..!) a bass..? There are very many splendid guitars, or all shapes and styles, for £100 or so, and ten times more to choose from up to £500. Does the brother have an 'guitar heroes' that he may like to emulate..?
When is 'soon'..? Time enough to order something on t'web and have it delivered..? Stuff from Europe may take a few days longer, but they have a wide range in their catalogues; there are several reputable on-line sellers in the UK, too (some with 'real shops' as well, such as Andertons; there are others still...).
Hmm... More questions than answers, I'm afraid, but I know my brothers better than I know yours, so it's not easy.
Fill in some of the gaps in our knowledge and we'll try to be more helpful still.