Have some experience although I am not a professional - you will probably get more replies if you ask MN to move this over to the Legal Matters board where there are a lot of discussion about family court matters (if you advanced search that board also I am sure you will find some international relocation threads).
Couple of things, I know you say your son has his father's surname but the important point is whether his father has parental rights, i.e. is on his birth certificate or not? If he doesn't then he'll have to go through the preliminary stage of asking the court to recognise his parental rights before he can object to any of the arrangements about his son.
In terms of the relocation, I know it's horribly stressful but I do think it's right that we have a legal system in this country that allows a parent to object to their other parent taking their child away permanently against their wishes. You're doing the right thing by asking for the father's permission and if he does object (which maybe isn't as certain as you think, if he's never had a relationship with your son and has been entirely absent from your lives for 7+ years now is he really going to create a load of hassle and expense just purely out of spite, I know there are men out there that are that vindictive but there are a lot more than will happily give it the big talk of 'pushing for full custody' or whatever but that when it comes down to it can't even be bothered to fill in the court forms never mind actually get a lawyer and turn up and fulfill their basic parental role, so you may be surprised. All the more so if by moving to Canada you're absolving him of any liklihood of having to pay child maintenance towards his son?
If it does come down to a contested hearing, the court is only concerned with your son's welfare and what's best for him, it's not an exercise in who's the better person between you and your ex or what caused your relationship to end or whatever. In deciding this they'll weigh up the pros and cons to your DS of a move to Canada (they will probably want Cafcass to talk to him to get an idea of his thoughts and views although he's too young of course for this to be the deciding factor), on the pro side they'll look at whatever you put forward in terms of the benefits of the move, better family support, better employment/financial situation, better educational or lifestyle opportunities for your DS etc, so it will be good if you're able to articulate these well, and of course they'll want to know you have a solid plan about where you'll live, what your and his immigration status in Canada will be, how you will fund yourself etc (I know this may seem obvious or even patronising but people do turn up at court asking permission for pie in the sky plans about moving their kids halfway across the world to places they have no right to reside in and no prospect of earning money to support themselves there, understandably the courts take a pretty dim view of this especially if the underlying reason for the move is to get the kids away from their father).
On the con side they'll consider the upheaval/disruption to your son of the move and having to change schools, leave behind friends etc (but this isn't in itself usually reason enough to refuse permission) and also they will likely think of it as a disadvantage that living in Canada will make it much harder for him to build a relationship with his father (and his father's extended family) in future should your son want that, particularly if the father is there in front of the judge passionately arguing that that is something he wants and is committed to.
Obviously I can't say for sure how the decision would fall in that instance, it would depend on all the facts and evidence, but in my opinion only, I think it's more likely than not you'd still be allowed to go. It's a very different thing where it's only a possible future relationship which is still untested and unproven compared to disrupting an existing parent/child bond, possibly also removing the child from half-sibling relationships or closeness to grandparents or other members of the father's family which I assume don't apply here? I have seen permission to relocate abroad granted even where there was a committed father with a full/real relationship with the child objecting, if there are strong enough reasons/benefits for the relocation.
If his father does turn up at court arguing he wants a relationship with his son, I think you would need to recognise the benefits to your son of at least knowing who his father is and having some contact, and to show you would encourage their relationship to develop as best you can from Canada. The judge might order a phased/gradual build up of a contact plan, starting with what they call indirect contact ie an exchange of letters or emails, photos, birthday cards, presents, school reports etc so your son starts to have an idea of who his father is and vice versa. If that goes well then having some phone calls or video calls, perhaps playing some age appropriate online games together, that kind of thing. Then eventually building up to face to face visits in Canada or the UK. It will probably immensely help your case if you can say you're open to working on something like that (you can have your private views on whether or not your ex will actually follow through on such a plan given the effort he'd have to put in, but don't voice those to the judge)...