I would say not to worry yet. My son had BIG issues with pronunciation - until about 3-4, milk was 'dok' and l/r/w were all pronounced 'w' until 5 or so. We did some practice at home of saying L but apart from that everything came right by itself..I think partly to do with learning to read and realising these letters had distinct sounds. His Rs have only come right since turning 6 - but now he's got no issues.