Ok if he's locking at the base of the neck, you really need to do some suppling exercises on him. My horse blocks at the poll AND the base of the neck, a horse needs to be able to go forward while also bending/flexing.
For starters, put him on a 20m circle in a good working trot, ask his jaw to come a little to the inside with your inside rein. When the jaw will give, put your inside leg on, take your inside rein a little more, and ask the inside hind to step slightly across to the outside of the circle, so that you are moving the quarters slightly out while asking for more bend in the poll/neck. Ask the jaw to move to the inside again, then let him straighten. Then ask again, this time asking for more bend to the inside. Then straighten. Continue this sort of work, letting him straighten when he gives to the bend, while also going forward off your leg. Increasing the degree of bend each time you ask. This will start to unlock his neck for you.
Think of the horse's shoulders as a lock, and the neck is the key. The lock is a bit stiff, so you have to wiggle the key a bit to unlock the lock/shoulders. Basically, you should be able to move the horse's head and neck anywhere that you like without the horse bracing against the hand. If you're travelling on a 20m circle to the left, try asking his jaw to come a little to the right and vice versa.
I warm my horse up by trying to move him every which way. We go shoulder in, travers, renvers, shoulder in, leg yield, shoulder in, travers etc. etc. I think ask him deep for half a circle, then bring him up, then deep again with significant flexion to the inside, then deep with significant flexion to the outside and so on.
Always moving the horse around, testing your contact, testing your submission all the while keeping the horse forward and up to the bridle. If you lose impulsion, don't release the bend and push the horse through until he gives. He must not learn to brace when he goes forward. A horse that is doing extended trot for example, should not be against the bridle, you should still be able to move the jaw, poll and neck without the horse leaning and bracing. So your horse can certainly travel forward and soft at the same time :)
Once he is forward and soft in the bridle, the muscle will start to come. But you'll never build topline if you horse has a block in the base of the neck
Hope this helps you a little, good luck!