- Concentrate on 'positive attention': spend as much fun time with him as you all can, encouraging and praising all good behaviour very clearly and consistently.
- Make sure he gets some stimulation during the day but not too much. That includes enough physical activity (getting rid of tension and energy) but keep loud music, busy environments, TV, etc to a minimum.
- What often works very well is if you can find an activity for him to do, which has a lot of rhythm or repetitive movement. This can help him find an alternative to the head banging, a more pleasant one:
Just sitting with him several times a day, while you hold and rock him is good. Do this when he is being a good boy (and do not save it for when he is having a tantrum or you will be compensating that undesired behaviour).
A rocking chair where he can sit and play at will is also great, encourage him to do that.
Or find some rhythmic music class for him? Or do lots of rhythmic songs together.
Or what can be great too is one of those indoor trampolines, with a bar to hold. You'll find one online or at a good local toy store. Important of course to make sure the trampoline can safely be used by him at his age and evidently supervised. IF he likes this, it can be a great way to get some exercise, get rid of some energy and with rhythmic movements.
- A last thing to do: this is the age where 'rules' become more and more in the picture: things that are allowed and things that aren't. It's about setting boundaries, not about strict discipline. Children need consistency, and boundaries help them understand the world around them. You have been doing that by consistently not allowing him in your bed, allowing milk at one awakening, ... Have consistent rules about what he can request when waking and what you simply will not allow during the night.
- The head banging itself: do not ignore completely but act in the spirit of all the above. Prevent him from injuring himself. But also don't make a big deal out of it (towards him). And: try to encourage the use of his stuffed animal, or a different one more. Give it to him when you sit and rock him during the day, hold it close, show and tell him how it is soft and is so nice to cuddle, ... basically you are showing him how to use it to self comfort. Then at night when he head bangs, pick up the animal and offer it to him, holding it close, ...
If you can work with the above during the day, that will help settle his behaviour. Remember though, as for all toddlers, for the largest part it is something they must indeed grow out of. But with gentle guidance, you can keep things pleasant most of the time.
Then about the night awakenings; pull the things you do during the day through to the night. Be consistent, applaud good behaviour, ... don't allow more requests than the ones you set. If you can avoid the milk at 4am that would be great - start with giving less, diluting it with water, or simply move to water. You can also give him a 'big boy's privilege' of having a cup next to his bed and being allowed to drink a sip without coming to you.
If at all possible, shift his bedtime to earlier by half an hour or a full hour. Such shifts in the schedule very often have a very good effect on the night awakenings.
And for all awakenings: keep them dim and dull at all times, do not leave his room with him or go straight back to his room asap if he came out. You may want to have the rocking chair in his room if it isn't there already.
Then for your husband: he is clearly having serious sleep deprivation effects. And it is crucial for him now to top up on sleep. As much as I know you also need your sleep, I do have the feeling you are coping better than him at the moment. Can you give him a week or two where he sleeps separately, with ear plugs if necessary? He may not want to do that out of worry and care for you, but from what you write it is really important for him to catch up, now.
Also napping at weekends and getting early nights can help a bit, but a week or two of full nights, can recharge so much. And he will then be able to help you more again.