I'd definitely recommend the whistle training. There is a sticky at the top of this part of the forum from Top Barks that gives great advice on this.
At around the same age, Henry's recall got quite poor. He had, like you say, learned he didn't always need to respond to 'come'. I started whistle training and had him on the long line for the last month of three months total training. The long line was essential for making sure we didn't 'break' the new recall. I tested it out against all his favourite distractions.
Henry is now 9.5 months old and in the last few weeks is back off the lead and his recall is very good. Not perfect yet. If he sees a dog he wants to play with, I can get him back if he has just set off, but if he's already playing, the whistle is no good. We are still working on that. I usually incorporate 10-15 minutes long line training into our long morning walk, just to keep fine tuning it.
The thing I would say about the whistle training is that it has improved Henry's behaviour off the lead at all times, not just when I blow the whistle. He will be foraging about in the woods and if I just call his name he comes barrelling out to find me (and get a biscuit). If he runs ahead, he will stop and wait where paths diverge to let me catch up and tell him which way we are going. The other day I met a friend in an open field. We were chatting, Henry was skipping around. He saw some dogs in the distance, moved towards them, then stopped and looked at me, waiting to be told if he could go or not. I can honestly say I've never seen him do that before! I think all the training that goes with the whistle also builds that relationship where it is the two of us in collaboration going for our walk. I only NEED to blow the whistle a few times, the rest of the time he is either with me or comes back just on the call of his name.