It certainly sounds like Woodstock is going through a fear stage at the moment. My understanding is that through puppyhood and as young adults dogs go through a few fear stages, but I'm not sure that you can say exactly when, more a case of recognising it when it happens. Another possibility that comes to mind is that he's become unsettled by staying away from home while you were on holiday?
I think if he was mine I'd start by not allowing people to approach or stroke him when you're out walking for the moment. I'd also be pro-active and ask other dog owners not to let dogs approach him if you think they are too big or bouncy for Woodstock to cope with. That isn't mollycoddling him, it's helping him to stay calm and hopefully regain his confidence as time goes by.
I know you're confident about his doggy day care but have you been able to observe Woodstock there recently? I'm not saying that day care has caused his fear, but if he's going through a fear stage it could be that he isn't as confident and happy in day care at the moment as he was before. Have you talked to the day care people about it and asked them if they've noticed any changes in him? Could you stay and observe him for a while one day, preferably without Woodstock knowing you're there and watching?
Could you put his crate next to your bed at night for the time being and see if that stops the night time howling? Once he starts settling through the night try moving the crate a bit further away gradually.
I think if he was mine I'd also take him to the vet for a check up in case there's anything hurting him and causing him to be fearful. Hopefully he'll be given a clean bill of health and then at least you'll know it's nothing physical.
Hope this helps
