Just to stick my two pennyworth in having recent experience of the DLA forms. Does the Medication ALWAYS work? If not ie. he still has night terrors etc however occasionally then when you fill in the forms you must place the emphasis on these such times. What I'm trying to say is that DLA is given for an extended period and who's to say that 3 months down the line you're not back at square one?
I was advised when filling mine in to focus on the "worst" of times and disregard the best. Hope this helps a bit. Remember, you are the only one who knows what you have to go through on those "worst" days and nights. x

Alot of people actually say focus on the worse case but this could possibly lead to claims of benefit fraud so you have to be very careful.
The advice would be to focus on your normal days and not your best or worst as even parents with children without needs can have bad nights so thats something to consider aswell.
As i always advise keep a diary for a minimum of 7 days even possibly 30 before a renewal and get an average and work with this.
So lets say out of 7 nights you were getting 5 nights good they would look towards this.
If out of 7 nights you get 3 good then look towards higher rate.
A diary would also be useful if you went to appeal.

And in regards to the 3 months down the line if things were to change then a change of circumstances would be recomended.