He has told me he doesn't expect me to wear a hijab and I never would.
Well he's already moved some pretty massive goalposts in your marriage so do you trust him not to move any more?
I strongly believe that you cannot just change your whole belief system like this unless it comes from a true desire to want to find an alternative path.
You are completely right, and anyone who cannot respect that view is a fool and a bully.
He has also said that the religion makes him a better husband to me, he has previously been quite aggressive towards me but that has since calmed down.
erm…I'm sorry wouldn't bank on it. If anything it's more likely to give him the justification for being a controlling aggressive bully, when you don't conform to his ideal of the good Muslim wife.
Even if I converted, would he change his controlling ways. No.
You see? You already know this. Don't walk into the trap PLEASE.
I think whatasoddingmess gives very good advice, BUT I think that recent converts to any religion (if by genuine personal choice and not prompted by someone else's expectations) are often, by nature, people who are prone to be very 'all or nothing' type people and will be drawn to more extreme interpretations and behaviours. You know about Ostentatious Consumption? Well I think they often catch Ostentatious Faith. They can often end up being among the most over-zealous, dogmatic, literal and inflexible of all. Just because whats DH is a well balanced and level-headed Muslim doesn't necessarily mean yours will be, and all evidence points so far to you being led like a lamb to the slaughter, so tread very carefully and don't allow yourself to be railroaded or emotionally blackmailed.
He's clearly already using religion to control you.
Although I don't agree that he has already mentally separated from you and is using Islam as an excuse to withdraw from the marriage. It sounds to me like he is withdrawing all intimacy as a way of emotionally blackmailing and controlling you. He hopes that you will start to feel so unhappy and rejected and fearful for the future of your marriage that you eventually give in. And that does not bode well for the future AT ALL.
And that Imam is a twat. He is filling your husband's head with emotive nonsense. Frankly, your husband sounds like a gullible fool with no mind of his own if he can't see through this - is he always this easily led?
Tell him it would be completely pointless and hypocritical to convert in name only just to appease some Imam when you don't actually feel any genuine calling or affinity with Islam. How does he think his God would feel about that? Or does his God not really care whether you believe or not, just so long as you buckle under the pressure to conform?
It sounds like he has you over a barrel I'm sorry to say. 
He has already reached a stage in his indoctrination that if you don't convert he may well divorce you the stupid man and if you do convert for a quiet life, he will start to demand and expect that you behave like his vision of a good Muslim wife, and if you don't then he will divorce you anyway.
Just remember that if you do convert you will throw away any autonomy in your marriage and your life. Only you know your DH's true character and only you can judge whether he is likely to use the tenets of Islam as a metaphorical stick to beat you with, but by the sounds of it he already is.
He will no doubt expect you to make some quite radical changes to the way you currently live, dress, behave, interact with non-blood related men, socialise and think. You may be ok with that, it may not be so very different to the way you live as a Sikh anyway (don't know much about Sikhism so couldn't really comment) or you could end up feeling very compromised, suffocated and oppressed by that indeed. My guess is the latter.
But you will not have a leg to stand on because in his eyes you will be a Muslim, you converted, and therefore you should just suck it up. I think you are between a rock and a hard place right now, I'm sorry to say.
If your DH was a previously a fairly well balanced and moderate man who loves you very much then you may be able to give him an ultimatum. He either accepts things as they are, or he has to choose between you and Islam. but be prepared for him to choose Islam.