I need to find appropriate vegan snacks for my grandson

Vegetable chips are available in every supermarket just as crisps are a lot of them are baked not fried. Aldi do hummus fries and pea fries you could serve with a veg dip made with vegan mayonnaise.

Carrot sticks and humous. Sliced peppers.

Sorry I was thinking savoury! Dried fruits fresh fruit such as watermelon with non dairy cream or yoghurt. Vegan chocolate vegan Peppa Pigs by M&S not too sweet.

Baby sweetcorns, carrot, cucumber and celery sticks.

Why are you looking for sweet treat ideas if he's already putting on weight with sugary snacks?

How old is he?

Oops! Me too.

Most supermarkets have fairly good ranges of vegan food available that is often found in the Free From area.
The sugary snacks not always what they seem either as many contain gelatin that is made from animal products.
You may well find that some of the best places for vegan food can be found online at least initially and then once you know what you are looking for, you can find in the supermarket.

There are numbers of no sugar recipes vegan recipes out there. One of my favourites are no sugar chocolate brownies made with stevia and sweet potatoes. Banana and chia seeds can be used as a substitute for eggs but I’m not sure of the quantities.

How about brain balls? These include dates, coconut, nuts and cacao.

Pinterest has lots of recipes as well.

Vegan sausages and sausage rolls are popular with our grandchildren. We always try to have some in the freezer.

I also make thin pancakes using soya milk and self raising flour, and fill them with all sorts of veggie mixtures.

Trail mix was very popular with our kids, look up how to mix, but really just a mix of dried fruit & nuts, including coconut shavings. Good because you can snack on how much you want and easy to mix up.

Why not fruit? If he is a bit fruit-averse (you don't say how old he is) try cubes of fruit, either on a skewer, or the end of a fondue stick, that he can dip in various sauces. I know you want to avoid sugary things, but I used to find that the small amount of melted chocolate (thin it down with coconut oil) or maple syrup was a 'gateway' to eating the fruit.

And tortilla chips with dips like hummus or salsa are great.

Definitely search for your local wholefood co-op / shop or similar where you can find good stuff and people to advise.

Simply Delish Jelly is Vegan. Sainbury stock it.I get mine from Amazon. There are also Simply Delish desserts but I haven't tried them yet.
I love Kind bars which are nearly all nuts ,they come in different flavours, I think they are Vegan but check first.

Sorry, I've just checked and Kind Bars are vegetarian but not vegan

Poor child.
Not sure which parent is vegan. Will there be dissent over his diet?
Are you sure it is sugary snacks that are causing weight gain, and is this weight gain unhealthy?

Bananas split in half longways and spread with peanut butter. ( I know, yuck but kids like it)
I’m sure if you google you’ll find lots of ideas.

Don't forget that many foods are calorie-rich, including dried fruit, nuts and some fruit.

Why not an apple or banana? Lots of biscuits you’d buy are vegan, but not great if you want to watch the calories. Oat and rice cakes are fairly low in calories and kids seem to like them. Aldi have some protein bars which have cashew nuts and dates in and are really nice and filling.

He is continually eating treats packed with sugar which are causing him to put on weight.
Can anyone suggest a sweet treat (not fruit) he might like

I'm confused.

Why do you want suggestions for sweet treats (not fruit) if you are concerned about him putting on a weight?
Try to give him healthy savoury snacks - that is if he does need to eat between meals because he's having a growth spurt.

How old is he? Is he having one of those slightly chubby phases before he shoots up in height or is he worryingly overweight?

My grandchildren like making vegan scotch pancakes or popcorn. I get them involved in the snack preparation.

Many vegetables are naturally sweet - but also nourishing. Strips of red pepper, little bowls of cooked whole green beans or peas and sweet potato chips are popular. Flavour Fusion garlic 'parmesan' is tasty on top and made from almonds!

I also make them smoothies (banana and strawberry's the favourite) and cocoa (with sugar - tut, tut) made with Oatly Barista.

My children would readily eat Puffed Wheat cereal without as a snack. It's 100% whole wheat and no added sugar or salt. Not to be confused with the variety coated with honey.

You can also make banana ice cream - the only ingredient is ripe banana - recipes on the internet.

Doh, correction, that should have read without milk.

My Gdcs (4 and 5) love dried mango, ditto dried apricots. I sometimes add them to an order from Holland and Barrett. Both very sweet - of course they both contain quite a lot of natural sugar.

Does he like nuts?

Tesco have 25 products in their "vegan ice cream" list on the website if that would suit him?
Different levels of sugar, you can check the percentages under "nutritional information."
There are some strawberry lollies with an ice cream centre he might like ?