Advice, Experiences etc would be a godsend from you please on this, sorry if it's a long read as well.

I have 3 dogs aged 13 yrs neutered, 5 years neutered & 9 months entire and one bitch aged 2 1/2 entire.  We're at the end of the second week of my girls season and my 5 year old has turned into a dog i don't recognise!!  We've always laughed at him because even when he was entire he was never interested in our girl when she was in season, we even called him the only gay in the village. 
The last couple of days have been a nightmare, he will not leave her alone (she doesn't mind one bit) and he's got chattering teeth, he seems to go into a trance and has started growling at our pup when he's in this state.  The change is dramatic, our pup and girl are seperated for obvious reasons.  My main worry is the growling at my pup as up until now they've got along fine.  There's be no actual fighting as our pup just walks away and comes to us.  It's like as if our 5yr old boy has turned into a Jekyll & Hyde dog.  It's really stressing me out.  I'm not a novice dog owner as we've always had 2 or 3 but i am a novice bitch owner, she's my first one apart from when i was a little girl.

Will my 5 yr old come back to being his old lovable self ever?

More than likely he will, but sadly you may have this problem every time she is in season and it could esculate.  I'm having to rehome my youngest boy due to serious problems when the girls are due in season and his temperament change to my older male.  They've lived together for almost 5 years with only the odd little scuffle around season time, but the last time it really esculated where I feel that the youngest one would of killed the other one had I not been around.  Any other time they were absolutely wonderful together and I could leave them at home together with no problems.

Thats awful for you, you must be going through hell, i have great sympathy with your situation. 
I've been totally knocked for six with this change in our boy as it was totally unexpected.  The situation isn't as bad as yours but like you've said i'm wondering what will happen next time.  I'm counting the days away for her to be finished.  We're lucky that we only have one bitch and she has her seasons every 8 months or so.  I'm trying to stay positive and playing it by ear and hoping that all gets back to normal in time.  My hubby has said he thinks it's nothing serious and we're more shocked because it's the first time there's been any sort of trouble between them. 

Sorry for waffling it's just i needed to know i'm not the only one in this situation.