Reconsidering this whole speech therapy thing?

When my son was 18 months he went in for a check up and was referred to a speech therapist. We decided not to take him to be evaluated and figured that since he was not yet 2 and the youngest of 3 and had an older brother that did most of his talking we would just work with him and see if he began to talk more. Well, his vocabulary has improved but here is the deal, he has to try really hard to put a sentence together and it will sound like this "Mommy Kick...Kickkick...hit..head " (Mommy, Kendrick hit me in the head.) He also has a hard time singing a simple song Twinkle Twinkle Little Star sounds like "Twinkle Twinkle eih uo sar oow ow uh uh eew eww are". Is this normal? He is able to express himself and tell you what he wants and his daycare provider does not seem to think he has a problem. Not to compare because I know that all children are different but my older two sons could speak in completely clear sentences by this age. Would you have him evaluated or do you think he is developing normally? He turned 2 in July.