Can I add too that apart from getting injured or killed (and yes, you MUST take him to the vet, please, they can probably recommend a good trainer whilst you're at it), if your dog gets out and is picked up by a member of the public he will end up in a council pound.
At best you will be charged for his release and per day that he's in there, you'll also get a serious bollocking and possibly be put under a Dog Control Order by the Dog Warden, worse still he might end up either:
In a pound which, when you contact them to ask if they have your dog, doesn't recognise the description/gets it all wrong because there are so many dogs in there/can't be arsed to check, and tells you that there is no such dog there. After 7 days as a picked up stray they are legally entitled to AND THEY DO "put him to sleep". Inverted commas used because as a rescuer I hate that saying, it ain't going to sleep, it's KILLING the dog!
Or taken to a vet by a concerned member if the public who thinks they are doing the right thing and being killed there (illegally, but that;s no help to you or your dog) because they have no room for him/don't want the hassle and expense and can't get the dog warden out quick enough to pick up. Trust me, vets like this exist, I've had run-ins with 2 such recently in my rescue work.
Or picked up by someone who wants to use him as a hunter or bait for fighting dogs or just to keep him.
Or who will claim/demand a "reward", often stating that they bought him from a stranger hundreds of miles away and that they need the hundreds of pounds they "paid" for him plus fuel costs to return him.
Or, if he's unneutered (IS he? That may be relevent), use him as a puppy farm stud.
Jackies are particularly desirable to certain sectors of the community for illegal hunting and he won't stand a chance if he strays and is picked up by a dodgy person... and there are far too many of those about, sadly.