android set custom font to a paint

If by “custom font” you mean a font that you are supplying as an asset, the following code should work:

Typeface plain = Typeface.createFromAsset(assetManager, pathToFont); 
Typeface bold = Typeface.create(plain, Typeface.DEFAULT_BOLD)
Paint paint = new Paint();
paint.setTypeface(bold);
canvas.drawText("Sample text in bold",0,0,paint);