If you want to do it go ahead but don't put pressure on him to unless he wants to.
Eat less and healthier and get moving exercise wise, faddy diets aren't good for you, can be dangerous, aren't sustainable and you tend to pile the weight back on after.
Get lots of fruit and salad and vegetables in and lean meat and fish, grill rather than fry and cut down on carbs, avoid lots of bread, pasta, potato etc... and drink lots of water.
Maybe suggest some activities you can do together which can help fitness without being focussed on fitness. Go for walks, swimming, ice skating etc...
If he sees you doing it and he isn't being nagged about it he might fancy joining it more.