I’m going to rock the boat here, and say that apart from showing the child where and how, and a little gentle encouragement, it’s best to leave them to do it in their own good time.
My children, all fit, healthy and bright, all graduates now, were TERRIBLY slow at potty training, and DGC are the same.
Late MIL pursed her lips and said all hers were clean and dry at six months (doubtful!) and recommended ‘a good thrashing’ 
2 DGDs responded eventually to a lovely book called ‘Princess Polly’s Potty’. There’s also ‘Pirate Pete’s Potty’ for little lads. It was great fun, with interactive sound effects. About £10 on Amazon, and well worth it for the fun, as well as it’s effectiveness.
Don’t worry about toilet training - a life time of neurosis isn’t worth it for a year of fewer nappies, so take it easy. Besides, looking at my brood, I can confidently say that being late in this department is an indicator of superior intelligence and ability! 