18 year old son with cp, dramatic mood swings

Hi, 2 winters ago my son suffered a form of what I thought was depression, not eating, not sleeping, very clingy, loss of interest in all the things close to him, it lasted from October - March, the winter months were long hard and a complete nightmare, this happened again last year, having researched SAD, I purchased a lamp and vitamin D, this helped no end and he came out of this 'mood' the end of march this year, however, nearly 4 weeks ago he's gone really done hill, anxious, not sleeping or eating and very grumpy and stubborn. I have done everything I can for him and as a last resort went to the gp who has prescribed him with anti anxiety tablets, they take the edge of my lads moods but not much else. My lad had cp, high learning disability, epilepsy and mild austism. Any help, views or suggestions please, I'm really flagging and don't know where to turn to. Thanks Sue